Don't cache exception values in the module cache

Exceptions hold tracebacks, which can get very large (especially for
highly recursive functions). Especially if the exceptions are being used
for control flow instead of actual exception management, we should be
throwing them away as fast as possible.

This commit deals with this with a bit of a hacky solution, but really
the way the module cache handles things ought to be reworked a bit.

astroid/manager.py

@@ -39,6 +39,24 @@ def safe_repr(obj):
         return "???"
 
 
+class AstroidImportError:
+    """
+    Error class to keep track of when exceptions happened.
+
+    This error class avoids holding onto the initial exception to avoid
+    memory usage explosion
+    """
+
+    def __init__(self, modname, error):
+        self.modname = modname
+        self.error = str(error)
+
+    def __str__(self):
+        return "AstroidImportError: Failed to import module {modname} with error:\n{error}.".format(
+            modname=self.modname, error=self.error
+        )
+
+
 class AstroidManager:
     """the astroid manager, responsible to build astroid from files
      or modules.
@@ -233,14 +251,12 @@ def file_from_module_name(self, modname, contextfile):
                     modname.split("."), context_file=contextfile
                 )
             except ImportError as ex:
-                value = exceptions.AstroidImportError(
-                    "Failed to import module {modname} with error:\n{error}.",
-                    modname=modname,
-                    error=ex,
-                )
+                value = AstroidImportError(modname=modname, error=ex)
             self._mod_file_cache[(modname, contextfile)] = value
-        if isinstance(value, exceptions.AstroidBuildingError):
-            raise value
+        if isinstance(value, AstroidImportError):
+            raise exceptions.AstroidImportError(
+                str(value), modname=value.modname, error=value.error
+            )
         return value
 
     def ast_from_module(self, module, modname=None):