errors: Rework error handling in main()

`main()` has grown, and in some cases code that can generate errors has gotten
outside the top-level try/catch in there. This means that simple errors like
config issues give you large stack traces, which shouldn't happen without
`--debug`.

- [x] Split `main()` into `main()` for the top-level error handling and
      `_main()` with all logic.

lib/spack/spack/main.py

@@ -727,12 +727,21 @@ def shell_set(var, value):
             shell_set('_sp_module_prefix', 'not_installed')
 
 
-def main(argv=None):
-    """This is the entry point for the Spack command.
+def _main(argv=None):
+    """Logic for the main entry point for the Spack command.
+
+    ``main()`` calls ``_main()`` and catches any errors that emerge.
+
+    ``_main()`` handles:
+
+    1. Parsing arguments;
+    2. Setting up configuration; and
+    3. Finding and executing a Spack command.
 
     Args:
         argv (list or None): command line arguments, NOT including
             the executable name. If None, parses from sys.argv.
+
     """
     # Create a parser with a simple positional argument first.  We'll
     # lazily load the subcommand(s) we need later. This allows us to
@@ -788,7 +797,6 @@ def main(argv=None):
         parser.print_help()
         return 1
 
-    try:
     # ensure options on spack command come before everything
     setup_main_options(args)
 
@@ -815,6 +823,23 @@ def main(argv=None):
     else:
         return _invoke_command(command, parser, args, unknown)
 
+
+def main(argv=None):
+    """This is the entry point for the Spack command.
+
+    ``main()`` itself is just an error handler -- it handles errors for
+    everything in Spack that makes it to the top level.
+
+    The logic is all in ``_main()``.
+
+    Args:
+        argv (list of str or None): command line arguments, NOT including
+            the executable name. If None, parses from sys.argv.
+
+    """
+    try:
+        _main(argv)
+
     except SpackError as e:
         tty.debug(e)
         e.die()  # gracefully die on any SpackErrors