diff --git a/lib/spack/llnl/util/lang.py b/lib/spack/llnl/util/lang.py
index 4913a50fad9..1cd7091aa3e 100644
--- a/lib/spack/llnl/util/lang.py
+++ b/lib/spack/llnl/util/lang.py
@@ -995,11 +995,8 @@ def _receive_forwarded(self, context: str, exc: Exception, tb: List[str]):
     def grouped_message(self, with_tracebacks: bool = True) -> str:
         """Print out an error message coalescing all the forwarded errors."""
         each_exception_message = [
-            "{0} raised {1}: {2}{3}".format(
-                context,
-                exc.__class__.__name__,
-                exc,
-                "\n{0}".format("".join(tb)) if with_tracebacks else "",
+            "\n\t{0} raised {1}: {2}\n{3}".format(
+                context, exc.__class__.__name__, exc, f"\n{''.join(tb)}" if with_tracebacks else ""
             )
             for context, exc, tb in self.exceptions
         ]
diff --git a/lib/spack/spack/bootstrap/clingo.py b/lib/spack/spack/bootstrap/clingo.py
index 337ac4c6f9c..4f381a5aeda 100644
--- a/lib/spack/spack/bootstrap/clingo.py
+++ b/lib/spack/spack/bootstrap/clingo.py
@@ -63,9 +63,15 @@ def _valid_compiler_or_raise(self):
             )
         candidates.sort(key=lambda x: x.version, reverse=True)
         best = candidates[0]
-        # FIXME (compiler as nodes): we need to find a better place for setting namespaces
+        # Get compilers for bootstrapping from the 'builtin' repository
         best.namespace = "builtin"
-        # TODO: check it has C and C++, and supports C++14
+        # If the compiler does not support C++ 14, fail with a legible error message
+        try:
+            _ = best.package.standard_flag(language="cxx", standard="14")
+        except RuntimeError as e:
+            raise RuntimeError(
+                "cannot find a compiler supporting C++ 14 [needed to bootstrap clingo]"
+            ) from e
         return candidates[0]
 
     def _externals_from_yaml(
diff --git a/lib/spack/spack/bootstrap/core.py b/lib/spack/spack/bootstrap/core.py
index 3d23ea24cc0..82c60b275ac 100644
--- a/lib/spack/spack/bootstrap/core.py
+++ b/lib/spack/spack/bootstrap/core.py
@@ -322,11 +322,10 @@ def create_bootstrapper(conf: ConfigDictionary):
     return _bootstrap_methods[btype](conf)
 
 
-def source_is_enabled_or_raise(conf: ConfigDictionary):
+def source_is_enabled(conf: ConfigDictionary):
     """Raise ValueError if the source is not enabled for bootstrapping"""
     trusted, name = spack.config.get("bootstrap:trusted"), conf["name"]
-    if not trusted.get(name, False):
-        raise ValueError("source is not trusted")
+    return trusted.get(name, False)
 
 
 def ensure_module_importable_or_raise(module: str, abstract_spec: Optional[str] = None):
@@ -356,8 +355,10 @@ def ensure_module_importable_or_raise(module: str, abstract_spec: Optional[str]
     exception_handler = GroupedExceptionHandler()
 
     for current_config in bootstrapping_sources():
+        if not source_is_enabled(current_config):
+            continue
+
         with exception_handler.forward(current_config["name"], Exception):
-            source_is_enabled_or_raise(current_config)
             current_bootstrapper = create_bootstrapper(current_config)
             if current_bootstrapper.try_import(module, abstract_spec):
                 return
@@ -369,11 +370,7 @@ def ensure_module_importable_or_raise(module: str, abstract_spec: Optional[str]
     msg = f'cannot bootstrap the "{module}" Python module '
     if abstract_spec:
         msg += f'from spec "{abstract_spec}" '
-    if tty.is_debug():
-        msg += exception_handler.grouped_message(with_tracebacks=True)
-    else:
-        msg += exception_handler.grouped_message(with_tracebacks=False)
-        msg += "\nRun `spack --debug ...` for more detailed errors"
+    msg += exception_handler.grouped_message(with_tracebacks=tty.is_debug())
     raise ImportError(msg)
 
 
@@ -411,8 +408,9 @@ def ensure_executables_in_path_or_raise(
     exception_handler = GroupedExceptionHandler()
 
     for current_config in bootstrapping_sources():
+        if not source_is_enabled(current_config):
+            continue
         with exception_handler.forward(current_config["name"], Exception):
-            source_is_enabled_or_raise(current_config)
             current_bootstrapper = create_bootstrapper(current_config)
             if current_bootstrapper.try_search_path(executables, abstract_spec):
                 # Additional environment variables needed
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index c6bafc48d75..47eb9c5d75a 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -5217,9 +5217,6 @@ class DuplicateCompilerSpecError(spack.error.SpecError):
 class UnsupportedCompilerError(spack.error.SpecError):
     """Raised when the user asks for a compiler spack doesn't know about."""
 
-    def __init__(self, compiler_name):
-        super().__init__("The '%s' compiler is not yet supported." % compiler_name)
-
 
 class DuplicateArchitectureError(spack.error.SpecError):
     """Raised when the same architecture occurs in a spec twice."""