Display correct error message for missing imports

Resolves django-extensions#955
- When a missing import is found in a script, this changes the error
message displayed to indicate that there's a missing import, rather than
giving a generic error message similar to the one raised when the script
does not exist

django_extensions/management/commands/runscript.py

@@ -107,7 +107,7 @@ def my_import(mod):
                     finally:
                         exc_traceback = None
 
-                if verbosity > 1:
+                if verbosity > 0 and not silent:
                     if verbosity > 2:
                         traceback.print_exc()
                     print(ERROR("Cannot import module '%s': %s." % (mod, e)))