Backport automation should report an error when expected format is not used

[RikkiGibson]

Example: dotnet/roslyn#78672 (comment)

It looks like Jared forgot the word "to", so, instead of a backport job starting, no response was given from the automation.

It feels like comments which start with /backport, but do not match the expected format, should cause the bot to reply indicating that there is a problem.

It looks like the automation is defined here: https://github.com/dotnet/arcade/blob/main/.github/workflows/backport-base.yml