Improve error from bad escapes in a toml config.

Whereas previously we would say:

target/debug/arti: error: invalid escape character in string: `Z` at line 9 column 14 in ../../.config/arti/arti.toml

we now say:

target/debug/arti: error: invalid escape character in string: `Z` at line 9 column 14 in ../../.config/arti/arti.toml   (If you wanted to include a literal \ character, you need to escape it by writing two in a row: \\)

The implementation is a bit of a hack, I'm afraid, but I don't think it's all that bad.

Closes #549 (closed).