mesonlib: Add an exception for what should be unreachable code

It gives us a little more saftey, and makes pylance happy.

mesonbuild/mesonlib/universal.py

@@ -1260,6 +1260,8 @@ def dump_conf_header(ofilename: str, cdata: 'ConfigurationData', output_format:
     elif output_format == 'nasm':
         prelude = CONF_NASM_PRELUDE
         prefix = '%'
+    else:
+        raise MesonBugException(f'Undefined output_format: "{output_format}"')
 
     ofilename_tmp = ofilename + '~'
     with open(ofilename_tmp, 'w', encoding='utf-8') as ofile: