provide more helpful error message when running an improperly-typed term

[samgqroberts]

Overview

Endeavoring to fix #1747 - see the output of the new transcript for what the new error message is and when it is shown.

Implementation notes

Instead of HandleInput.addRunMain just returning a Maybe (I could do it, or I couldn't), it now returns a type that disambiguates whether it couldn't due to not being able to find the term, or whether the term has a bad type. That new case is subsequently handled in the error reporting module OutputMessages.hs to produce the new error message.

Interesting/controversial decisions

Test coverage

I've included a new transcript run.md that attempts to run properly-typed, improperly-typed, and nonexistent terms to show the differences in behavior / error messages.

Loose ends

• This PR is obfuscated by another bug - you'll see me call this out in the new transcript. I believe I'm running into Regression in run - fails to find the function to run #1800, and I've confirmed that without my changes that runnable function still fails to be found. I wanted to put this PR up optimistically to gather feedback while that issue is resolved.
• I didn't update the error reporting in Execute.hs to provide this distinction (partly due to my lack of familiarity of what experience that file is part of..)

In general, this being my first contribution, I'm not confident at all that the changes I've provided are idiomatic / sufficient. I'd be more than happy to add tests, move code, rename types. Thank you!

[pchiusano]

This is great, a nice little usability improvement! Can you add yourself to CONTRIBUTORS.markdown? I'll wait for CI to pass before merging.

[aryairani]

LGTM