main: return exit code 2 when an input is not found

This commit makes skopeo return a different exit code when an
input is not found. The use case is `osbuild` which uses skopeo
to inspect images and it would be nice to differenciate between
an image that is not found and general skopeo errors (or errors
like network issues etc).

I picked exit code `2` for `not found` because it is also the value
of `ENOENT`.

Man page and a test are added.

Signed-off-by: Michael Vogt <mvogt@redhat.com>

systemtest/040-local-registry-auth.bats

@@ -40,7 +40,7 @@ function setup() {
     expect_output --substring "authentication required"
 
     # Correct creds, but no such image
-    run_skopeo 1 inspect --tls-verify=false --creds=$testuser:$testpassword \
+    run_skopeo 2 inspect --tls-verify=false --creds=$testuser:$testpassword \
                docker://localhost:5000/nonesuch
     expect_output --substring "manifest unknown"