feat($compile): throw error when directive name or factory fn is invalid

[Narretz]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
feat

What is the current behavior? (You can also link to an open issue here)
when the directive name or factory fn is undefined, no error is thrown

What is the new behavior (if this is a feature change)?
better errors are thrown

Does this PR introduce a breaking change?
No. Although I'm not sure if isFunction will catch all exotic ways to define a function.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)

Other information:

Closes: #15056

[gkalpak]

If we do this for directives, why not do the same for controllers, filters, services etc? I feel we are going to far trying to be helpful here.

[Narretz]

Is there a thing as too helpful? :D
The difference between directives and controllers, filters, services is that if the latter haven't been registered, they will complain when they are actually used. Directives are just attributes in DOM elements, so if there's no match than just nothing happens.
I think it's a change we can make - it's very little code.

[gkalpak]

Is there a thing as too helpful? :D

Absolutely 😛

The difference between directives and controllers, filters, services is that if the latter haven't been registered, they will complain when they are actually used.

Point taken 😄
I don't find this necessary, but since it's such a small change I am OK merging this (as long as this not a considered a breaking change).

[Narretz]

I don't think it's a breaking change, as I can't see a use case for adding an "empty" directive. And functions should be detected regardless of which shape they are. I guess I was a bit worried because we had that issue with the constructor fns in compile, but this is something different.

[Narretz]

I've relaxed the check for the factory, as we can't guarantee 100% that our isFunction detects the most exotic ways of defining a function.