cli: tell how many messages were precisely matched when expected 1 match

In case of notmuch reply and notmuch show --part=N it is required that
search terms match to one message. If match count was != 1, error
message "Error: search term did not match precisely one message"
was too vague to explain what happened.

By appending (matched <num> messages) to the error message it
makes the problem more understandable (e.g when <num> is '0'
user reckons the query had a typo in it).

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