git-svn: match SVN 1.5 behaviour of info' on unknown item

git-svn: match SVN 1.5 behaviour of info' on unknown item

Previously 'git svn info unknown-file' only announced its failure (in
the SVN 1.4 style, "not a versioned resource"), and exited
successfully.

It is desirable to actually exit with failure, so change the code to
exit(1) under this condition.  Since that is already halfway SVN 1.5
compatibility, also change the error output to match 1.5.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Eric Wong <normalperson@yhbt.net>