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

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

lib/python/qmk/cli/c2json.py

@@ -9,6 +9,7 @@
 import qmk.path
 from qmk.json_encoders import InfoJSONEncoder
 from qmk.keyboard import keyboard_completer, keyboard_folder
+from qmk.errors import CppError
 
 
 @cli.argument('--no-cpp', arg_only=True, action='store_false', help='Do not use \'cpp\' on keymap.c')
@@ -37,7 +38,13 @@ def c2json(cli):
         cli.args.output = None
 
     # Parse the keymap.c
-    keymap_json = qmk.keymap.c2json(cli.args.keyboard, cli.args.keymap, cli.args.filename, use_cpp=cli.args.no_cpp)
+    try:
+        keymap_json = qmk.keymap.c2json(cli.args.keyboard, cli.args.keymap, cli.args.filename, use_cpp=cli.args.no_cpp)
+    except CppError as e:
+        if cli.config.general.verbose:
+            cli.log.debug('The C pre-processor ran into a fatal error: %s', e)
+        cli.log.error('Something went wrong. Try to use --no-cpp.\nUse the CLI in verbose mode to find out more.')
+        return False
 
     # Generate the keymap.json
     try:

lib/python/qmk/errors.py

@@ -3,3 +3,10 @@ class NoSuchKeyboardError(Exception):
     """
     def __init__(self, message):
         self.message = message
+
+
+class CppError(Exception):
+    """Raised when 'cpp' cannot process a file.
+    """
+    def __init__(self, message):
+        self.message = message

lib/python/qmk/keymap.py

@@ -13,6 +13,7 @@
 
 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