migrations: loader: warn in case an explicit migration module is not found

[blueyed]

Logging a warning in this case helps with tracking down errors where you
have an invalid entry in MIGRATION_MODULES.

Does this make sense?
Is it OK to add logging here?

[timgraham]

I think this is somewhat dependent on whether or not we require migrations in 1.9. Is there a reason not to raise an error in this case? Possibly a system check could be added for invalid MIGRATION_MODULES entries, though if migrations are required for apps, the issue seems obsolete. I added a link to this PR to https://code.djangoproject.com/ticket/25109.

[blueyed]

Is there a reason not to raise an error in this case?

Would be good, too - given that there will be an (obfuscated) error later already (InvalidBasesError).

I think a combination of this PR and the patch at from https://code.djangoproject.com/ticket/25109 might be good: do not add it to self.unmigrated_apps and output a warning.
A system check might be good, too - but it might not be feasible to check this always?!

though if migrations are required for apps, the issue seems obsolete

Why?

[timgraham]

Re: "Why?" -- If migrations are required, I guess "unmigrated apps" would go away and there would be an exception for apps missing migrations. Obsolete in the sense of "if we have an exception, we don't need a warning".

[blueyed]

Obsolete in the sense of "if we have an exception, we don't need a warning".

Yes, but the error would be still clearer if there was an additional warning or more explicit error about the root cause.