branch: change the error messages to be more meaningful

branch: change the error messages to be more meaningful

The error messages shown when the branch command is misused
by supplying it wrong number of parameters wasn't meaningful.
That's because it used the the phrase "too many branches"
assuming all parameters to be "valid" branch names. It's not
always the case as exemplified below,

        $ git branch
          foo
        * master

        $ git branch -m foo foo old
        fatal: too many branches for a rename operation

Change the messages to be more general thus making no assumptions
about the "parameters".

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>