Make invalid type ignore comments non-blocking

Blocking errors are a bad user experience and there's no reason for this
one to be one / there is another code path for invalid type ignores that
is non-blocking.

Fixes half of python#12299. The half it doesn't fix is that ideally users
shouldn't be getting these warnings from third party libraries.
Also see python#12162 (comment)
But that's for another PR

mypy/fastparse.py

@@ -828,7 +828,7 @@ def visit_Module(self, mod: ast3.Module) -> MypyFile:
             if parsed is not None:
                 self.type_ignores[ti.lineno] = parsed
             else:
-                self.fail(INVALID_TYPE_IGNORE, ti.lineno, -1)
+                self.fail(INVALID_TYPE_IGNORE, ti.lineno, -1, blocker=False)
         body = self.fix_function_overloads(self.translate_stmt_list(mod.body, ismodule=True))
         return MypyFile(body, self.imports, False, self.type_ignores)