Admin endpoint for mocks creation should return 400 if the input is not valid

[zhouzhuojie]

I think a few functions may need to bubble up the errors so that we can prevent invalid input early on. Incidents like an invalid yaml input could potentially crash the openmock server and may need manual interaction from the redis server.

https://github.com/checkr/openmock/blob/68fadf576ac767f742206f76c0dd91d3d6f91b16/pkg/admin/convert.go#L62

[zhouzhuojie]

cc @sesquipedalian-dev @elan-devops