Show qualified backend error message on SMUI frontend (for BadRequest/400)

[pbartusch]

Follow-up on #52

In case the backend API delivers a BadRequest/400 an "unexpected" error is reported to the user of SMUI.

Example: Create empty spelling / rule input.

In those cases the errors, like validation errors, should be expected by SMUI's frontend and reported including the specific error message delivered by the backend.