Consider providing a standard way for formatters to report errors

Currently, if formatters want to report errors, they update the quickfix list directly. (As shipped, only the gofmt formatter does this, but it would also make sense for e.g. google-java-format, were we to add support for that.) It would be nice to make it easier for formatters to report errors.

Additionally, we have some questions around when formatters should clear the quickfix list (e.g. #58), and I'm not sure how stacked formatters (#44) would work if formatters were managing the quickfix list individually.

I've been wondering whether formatters could pass a structured error list to codefmt somehow, and then codefmt itself could be responsible for merging/populating/hiding the quickfix list as appropriate, consistently.

As a strawman, perhaps `FormatRange()` (etc) could return a list of (line, column, message)? (Or accept a callback instead, if that sounds like a restrictive API? Would the line numbers be relative to the start of the range, for range formatting?)

And/or, since most formatters can get errors by looking for "file:line:column: error" on stderr, it might even be work providing a helper that handled exactly that pattern directly.