Error highlighter misbehaves in the presence of tabs

[jdm]

The calculations for ^~~~~~~ treat tabs as one character, but that displays incorrectly.

[catamorphism]

7fe6b1b should fix this, but I'd like to test it manually before closing the bug. (I'm not sure how to test this issue with a regression test.)

[Aatch]

Closing this as the spans match up now.