add field error_type (enum) to struct hs_compile_error

I want to detect and handle "Pattern is too large" errors. The only option I have now is to look for string "Pattern is too large" in `hs_compile_error.message`, but it is a poor solution. Could you provide a `enum` with error types and add it to `struct hs_compile_error`, please? Description of `hs_compile_error` has a list of possible problems, it can be converted to the `enum`.