Return DimensionMismatch rather than ErrorException

[timholy]

Seems reasonable to use the "official" exception type here. We don't seem to have a specific test for this (it's probably a @test_throws Exception ... test); I can make it more specific if desired.

[coveralls]

Coverage remained the same at 66.125% when pulling 4ac3e4e on teh/dm into a46d711 on master.

[andyferris]

Thanks. I’m notoriously bad at this, we should probably clean this up around the place.

[timholy]

Want me to make the test more specific to lock this in place? Say the word and I'll do it.

Will want this tagged at some point for JuliaImages/ImageTransformations.jl#63

[andyferris]

Yes - you’re of course very welcome to help out with this stuff! ;)

(To expand on my earlier response, I find that I often use return types in my code to handle the problems that I want to handle, and generally expect exceptions to exit. I’m frequently left with no idea what exception type to return because I can’t imagine the case I would catch these kinds of exceptions - ones like this tend to result from code bugs rather than input data, or interaction with live systems, or whatever).

[timholy]

95% of the time I use the exception type only for tests, to make sure it's failing the way I intend it to rather than for a far stupider reason. One of my greatest coding skilz is to introduce a typo in an error branch and getting a MethodError or UndefVarError while en route to what I hoped would throw a DimensionMismatch. Oops.