Clean up `TemplateSummaryError` error formatting

In `api/Error.go`, we are using `\n` for error formatting in a few places, when errors are usually expected to be single line return values. For example here: https://github.com/influxdata/influx-cli/blob/041ebf65e336632f2fe4028396f5574238b6b28b/api/error.go#L121 (this code has changed very slightly since this permalink, but the result is the same).

@lesam suggested using a tool like https://github.com/hashicorp/go-multierror instead to clean this up

Start in api/Error.go, especially the TemplateSummaryError formatting referenced by the issue, and inspect the other places using \n in errors. Review whether github.com/hashicorp/go-multierror fits the cleanup; done means error returns are single-line or otherwise consistently formatted without the current newline behavior.

Written by the indexing model from the issue text.