Improve error message when using a wrong Content-Type on container creation

This is a proposal for improving error message on container creation, as requested in #2515

As a first contribution, I'd be happy if someone told me:
- if this should be put elsewhere ? (for instance factored in `makeHttpHandler` ?)
- if this can be tested easily ; I didn't found tests for error messages in `api/server/server_unit_test.go` but if you have any pointer I'd be happy to add unit tests for this...

Thanks