ui: include error sources in pager warning

[yuja]

Extracted from #4203

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[ilyagr]

I guess you are doing this for parity with other errors jj prints? Or do you know a case where it makes a difference?

In any case, it can't hurt.

[yuja]

I guess you are doing this for parity with other errors jj prints? Or do you know a case where it makes a difference?

MinusError is now printed as "Failed to initialize the terminal: The standard output is not a valid terminal", etc. It's not super important here, but sometimes the outermost error message is useless.