ssh: handle FileNotFoundError errors

If ssh isn't installed, it throws a distinct error we have to catch.

Bug: https://crbug.com/gerrit/15196
Change-Id: I0660e842c304ce7575f5cb100894d05fd65f9454
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/322055
Reviewed-by: Jack Neus <jackneus@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/ssh.py b/ssh.py
index 0ae8d12..450383d 100644
--- a/ssh.py
+++ b/ssh.py
@@ -52,6 +52,9 @@
   """return ssh version as a tuple"""
   try:
     return _parse_ssh_version()
+  except FileNotFoundError:
+    print('fatal: ssh not installed', file=sys.stderr)
+    sys.exit(1)
   except subprocess.CalledProcessError:
     print('fatal: unable to detect ssh version', file=sys.stderr)
     sys.exit(1)