textformatter: errors should be in quotes like strings are

To be parsable by `logfmt` parsers, the errors should be wrapped within quotes.

`logfmt` is the format `l2met` is based on. See:

* Article: https://brandur.org/logfmt
* Go parser: https://github.com/kr/logfmt

@Sirupsen 
nope, `fmt` does it