1.4.x doesn't give friendly error for invalid tag on push

[dtrudg]

In our automated additional QA for SingularityCE & PRO, CE with scs-library-client 1.4.1 appears to attempt to push an image with an invalid tag (to SCS staging) and fails with an unclear error.

This is a regression from failing fast with a message that the tag is invalid.

=== RUN   TestInvalidPushDest/invalid-tag
    push_test.go:46: expected message ["Provided tag is not valid" "invalid tag: t3$t!"] was not found in output: WARNING: Skipping container verification
        INFO:    Uploading 782336 bytes
        FATAL:   Unable to push image to library: error uploading index