If a module build is triggered with --optional side_tag=, and identifier being something made-up - MBS doesn't fail the build and the buid hangs in wait state.

Threre's a trace visible in MBS logs:

Traceback (most recent call last):
File "/usr/lib/python2.7/site-packages/celery/app/trace.py", line 382, in trace_task
File "/usr/lib/python2.7/site-packages/celery/app/trace.py", line 641, in __protected_call__
R = retval = fun(args, **kwargs)
return self.run(args, **kwargs)
File "/usr/lib/python2.7/site-packages/module_build_service/scheduler/events.py", line 80, in wrapper
builder.buildroot_add_repos(build_deps)
File "/usr/lib/python2.7/site-packages/module_build_service/scheduler/handlers/modules.py", line 395, in wait
return func(args, **kwargs)
File "/usr/lib/python2.7/site-packages/module_build_service/builder/utils.py", line 339, in wrapper
self._koji_add_many_tag_inheritance(self.module_build_tag, koji_tags)
File "/usr/lib/python2.7/site-packages/module_build_service/builder/KojiModuleBuilder.py", line 480, in buildroot_add_repos
File "/usr/lib/python2.7/site-packages/module_build_service/builder/KojiModuleBuilder.py", line 800, in _koji_add_many_tag_inheritance
return function(args, **kwargs)
parent = self._get_tag(parent)
return function(*args, **kwargs)
File "/usr/lib/python2.7/site-packages/module_build_service/builder/utils.py", line 339, in wrapper
"/usr/lib/python2.7/site-packages/module_build_service/builder/KojiModuleBuilder.py", line 779, in _get_tag
SystemError: Unknown tag: module-rhel-8.3.0-z--build
raise SystemError("Unknown tag: %s" % tag)