More descriptive error when package constructor fails.

- helps package_sanity test identify which package failed.
- encountered while upgrading versions in Adam's packages to the new format.

lib/spack/spack/packages.py

@@ -69,7 +69,10 @@ def get(self, spec):
 
         if not spec in self.instances:
             package_class = self.get_class_for_package_name(spec.name)
+            try:
                 self.instances[spec.name] = package_class(spec)
+            except Exception, e:
+                raise FailedConstructorError(spec.name, e)
 
         return self.instances[spec.name]
 
@@ -227,3 +230,12 @@ class UnknownPackageError(spack.error.SpackError):
     def __init__(self, name):
         super(UnknownPackageError, self).__init__("Package %s not found." % name)
         self.name = name
+
+
+class FailedConstructorError(spack.error.SpackError):
+    """Raised when a package's class constructor fails."""
+    def __init__(self, name, reason):
+        super(FailedConstructorError, self).__init__(
+            "Class constructor failed for package '%s'." % name,
+            str(reason))
+        self.name = name