Extend signature error handling to compile-time errors

[barefootcoder]

In issue #54, @larryl gave us a way for subclasses to change the default behavior of run-time errors. In that issue, @schwern asked:

Perhaps it can be extended to cover other error messages we have?

We only have three calls to die in the module (not counting the one in Method::Signatures::Modifiers, which I think is a separate issue):

1. run-time errors
2. compile-time errors
3. pre-existing errors at parse time, which we rethrow

So, 1) is handled by #54. 3) we should probably not pass to our subclasses (given that it doesn't come from us). So that just leaves 2).

Compile-time errors are handled by sig_parsing_error in Method::Signatures::Parser. We could just change that die to a call to signature_error_handler and call it a day. However, there are three considerations that I'd like to clear up first.

• It means passing $class down into some places where it currently isn't. Probably not a big deal, but a bit of extra work.
• Just because we could doesn't mean we should. Should we, in fact, allow subclasses to grab compile-time (i.e. signature parsing) errors?
• Assuming we should, should we then modify signature_error_handler to be able to distinguish run-time from compile-time errors? I can imagine a subclass wanting to handle those differently.

I'm going to go ahead and get the fix for #54 live, so this is not a gating factor or anything. But I'd like some further discussion before proceeding with adding compile-time errors to this.