Fix mypyc regression with pretty (#16124)

Fixes #15877

Regression was introduced by #15070. Previously Errors objects created
in mypyc build would just use all the default values, now they use the
actual options object involved

Author: hauntsaninja

mypy/errors.py

@@ -909,8 +909,7 @@ def file_messages(self, path: str) -> list[str]:
             return []
         self.flushed_files.add(path)
         source_lines = None
-        if self.options.pretty:
-            assert self.read_source
+        if self.options.pretty and self.read_source:
             source_lines = self.read_source(path)
         return self.format_messages(self.error_info_map[path], source_lines)