Clean up error definitions, location and expose them in the headers

- Exposes all errors in the header file so any user of the api can test
   for the specific error conditions
 - Mark all static error pointers as const
 - Move generic errors into error.h
 - Name all errors err_modulename_* for errors that belong to a specific
   module and err_* for generic errors.

src/parser/util.c

@@ -1,7 +1,7 @@
 #include "util.h"
 #include "../tokenlist.h"
 
-error_t *err_parse_no_match =
+error_t *const err_parse_no_match =
     &(error_t){.message = "parsing failed to find the correct token sequence"};
 
 parse_result_t parse_error(error_t *err) {