Clean up `TemplateSummaryError` error formatting

[serenibyss]

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