minor cleanup of DimensionMismatch error

* change field from name -> msg to align with other error types
  The majority of DimensionMismatch errors do not return the original
  methodname so this is a bit of a misnomer
* change type of msg from ASCIIString -> AbstractString (closes #8984)
* Add convenience constructor `DimensionMismatch()` for the common case
  where no err msg is provided
Merge pull request #9226 from JuliaLang/jcb/dimerror

minor cleanup of DimensionMismatch error