Expose DynamicProxy exception messages when fake creation fails

[blairconrad]

Spun off from #157, whose fix will make use of this enhancement. Read it for probably more history than you want.

FakeItEasy goes to a fair amount of trouble to help provide useful error messages to users. Mostly this works. Sometimes, we end up losing information that comes from DynamicProxy that would help the users. Let's not do that. In additions to the "faking internal interfaces" issue, this would help with such errors as "provided constructor arguments are no good". It also opens the door to helping improve other bad error messages, possibly while improving DynamicProxy at the same time.