Gracefully handle invalid directives

[DrewML]

Fix as discussed in #39. Let me know if you want to change the copy used in the warning.

Issue can be replicated with this plnkr.

This fix warns the user, and prevents the inspector from failing.

[DrewML]

@rev087 Is invoke[2][0] in this context always === the name of the directive? I just realized it may be more useful to include the directive name in the warning.

[rev087]

this.name contains the name of the directive, if it is defined

[rev087]

Adding the name is definitely important, just remember to update the test (I'm monkeypatching the console.warn call and checking for the whole string in the assert, but a substring should suffice since the string will change with the name).