Improve has_expr messaging

[filipsch]

• Currently, it's showing an 'expected x but got y'-like message, even though these x and y can be huge lists. If they aren't simple strings, they should be summaries instead:
  • "Expected a list with 5 elements, but a list with 4 elements".
• If name is set, the feedback message doesn't necessarily make sense.
• If context values have been specified, the feedback messages don't necessarily make sense.
• If environment variables have been specified, the feedback messages don't necessarily make sense.

[filipsch]

Partly done, other part is in #317