Improve validation error messages

[morremeyer]

Is your feature request related to a problem? Please describe.
there is no category matching your query is technically correct, but we can definitely produce better error messages in the different cases:

• When creating an envelope, the message should be more like "you need to specify a category"
• When updating it, the message should be more like "the category you're trying to move this envelope to does not exist"

Of course, this applies to all resources, not only envelopes.