Error message for invalid shift creation attempt

[eliblock]

When a user attempts to create a shift for a zone to which they don't have access or a time that is not allowed, an error message should be displayed.

Currently users can access invalid zones or times by clicking on the location name (see image). When a new shift is submitted the request does not go through, but currently there is no feedback to the user explaining that the shift creation was invalid (and why).