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/lexer.h

@@ -5,7 +5,10 @@
 #include <stddef.h>
 #include <stdio.h>
 
-extern error_t *err_eof;
+extern error_t *const err_lexer_already_open;
+extern error_t *const err_prefix_too_large;
+extern error_t *const err_buffer_underrun;
+extern error_t *const err_consume_excessive_length;
 
 typedef enum {
     TOKEN_ERROR,