`custom_error_handler` needs redesign

[giordano]

While trying the reproduce the issue reported in #63 I've noted that custom_error_handler needs a redesign. This is its current signature:

custom_error_handler(reason::AbstractString, file::AbstractString, line::Integer, errno::Integer)

but some of the errors to be thrown needs more information, like DomainError(). BTW, custom_error_handler is actually never used directly but always through GSL_ERROR, which hides all arguments but errno. Thus, probably also GLS_ERROR needs some redesign.

PS: as a general comment, this package really needs some serious improvement in test coverage. There are really old cruft in the code, like calls to Compat.@dep_vectorize_2arg, I'm even surprised this macro is still in Compat.