git-cvsimport.perl: Bail out right away when reading from the server fails

git-cvsimport.perl: Bail out right away when reading from the server fails

If the CVS server is down, this reduced the git-cvsimport output from:

ssh: connect to host ijbswa.cvs.sourceforge.net port 22: Connection refused
Use of uninitialized value $rep in scalar chomp at /usr/local/libexec/git-core/git-cvsimport line 369.
Use of uninitialized value $rep in substitution (s///) at /usr/local/libexec/git-core/git-cvsimport line 370.
Expected Valid-requests from server, but got: <unknown>

to the less noisy:

ssh: connect to host ijbswa.cvs.sourceforge.net port 22: Connection refused
Failed to read from server at /usr/local/libexec/git-core/git-cvsimport line 370.

In this case a silent exit() instead of the die() would probably do,
but I assume that there could be cases where the connection attempt
succeeds, but reading from the server fails for other reasons.

Signed-off-by: Fabian Keil <fk@fabiankeil.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>