CLI/c2json: Print 'cpp' error when executed in verbose mode (#12869)

Co-authored-by: Ryan <fauxpark@gmail.com>

lib/python/qmk/keymap.py

@@ -13,6 +13,7 @@ from pygments import lex
 
 import qmk.path
 from qmk.keyboard import find_keyboard_from_dir, rules_mk
+from qmk.errors import CppError
 
 # The `keymap.c` template to use when a keyboard doesn't have its own
 DEFAULT_KEYMAP_C = """#include QMK_KEYBOARD_H
@@ -372,7 +373,10 @@ def _c_preprocess(path, stdin=DEVNULL):
     """
     cmd = ['cpp', str(path)] if path else ['cpp']
     pre_processed_keymap = cli.run(cmd, stdin=stdin)
-
+    if 'fatal error' in pre_processed_keymap.stderr:
+        for line in pre_processed_keymap.stderr.split('\n'):
+            if 'fatal error' in line:
+                raise (CppError(line))
     return pre_processed_keymap.stdout