`custom_error_handler` needs redesign

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