improve TB MAR signing process when incorrect password entered

Currently, if an incorrect NSS password is entered during the MAR file signing process, no error is reported and the NSS "get password" function consumes all available CPU. I believe this happens because NSS continues to try to read a password (retry) even though stdin has been closed.

gk encountered this problem when generating some signed MAR files for testing. We should either fix the underlying bug or at the very least change signmars.sh to provide more feedback during the signing process (so the fact that no real progress is being made is obvious). gk said:

"Maybe adding some echo commands just saying something like "Starting the signing process..." and "Signed MAR file $COUNT." would already be enough?"