Minor fixes in parallel checking (#21319)

Not adding tests for these fixes, as they are covered by running
`cmdline` tests in parallel mode.

Author: ilevkivskyi

mypy/build_worker/worker.py

@@ -48,6 +48,7 @@
 )
 from mypy.cache import Tag, read_int_list, read_json
 from mypy.defaults import RECURSION_LIMIT, WORKER_CONNECTION_TIMEOUT, WORKER_IDLE_TIMEOUT
+from mypy.error_formatter import OUTPUT_CHOICES
 from mypy.errors import CompileError, ErrorInfo, Errors, report_internal_error
 from mypy.fscache import FileSystemCache
 from mypy.ipc import IPCException, IPCServer, ready_to_read, receive, send
@@ -319,24 +320,27 @@ def flush_errors(filename: str | None, new_messages: list[str], is_serious: bool
         # We never flush errors in the worker, we send them back to coordinator.
         pass
 
-    return BuildManager(
-        data_dir,
-        search_paths,
-        ignore_prefix=os.getcwd(),
-        source_set=source_set,
-        reports=None,
-        options=options,
-        version_id=__version__,
-        plugin=plugin,
-        plugins_snapshot=snapshot,
-        errors=ctx.errors,
-        error_formatter=None,
-        flush_errors=flush_errors,
-        fscache=ctx.fscache,
-        stdout=sys.stdout,
-        stderr=sys.stderr,
-        parallel_worker=True,
-    )
+    try:
+        return BuildManager(
+            data_dir,
+            search_paths,
+            ignore_prefix=os.getcwd(),
+            source_set=source_set,
+            reports=None,
+            options=options,
+            version_id=__version__,
+            plugin=plugin,
+            plugins_snapshot=snapshot,
+            errors=ctx.errors,
+            error_formatter=None if options.output is None else OUTPUT_CHOICES.get(options.output),
+            flush_errors=flush_errors,
+            fscache=ctx.fscache,
+            stdout=sys.stdout,
+            stderr=sys.stderr,
+            parallel_worker=True,
+        )
+    except CompileError:
+        return None
 
 
 def console_entry() -> None:

mypy/options.py

@@ -479,8 +479,13 @@ def process_error_codes(self, *, error_callback: Callable[[str], Any]) -> None:
         if invalid_code_names_here:
             error_callback(f"Invalid error code(s): {', '.join(sorted(invalid_code_names_here))}")
 
-        self.disabled_error_codes |= {error_codes[code] for code in disabled_code_names}
-        self.enabled_error_codes |= {error_codes[code] for code in enabled_code_names}
+        # Ignore invalid error codes.
+        self.disabled_error_codes |= {
+            error_codes[code] for code in disabled_code_names if code in error_codes
+        }
+        self.enabled_error_codes |= {
+            error_codes[code] for code in enabled_code_names if code in error_codes
+        }
 
         # Enabling an error code always overrides disabling
         self.disabled_error_codes -= self.enabled_error_codes