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/ast.h

@@ -7,6 +7,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+extern error_t *const err_ast_children_cap;
+
 typedef enum node_id {
     NODE_INVALID,