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 
Do we need to call `.Error()` on the error to make sure we get the string, or is `fmt` smart enough to do that?