scripts/kernel_bump: Improve omitted version error

If a version string was not supplied, we currently print an empty
string. We can do better here. Also by popular demand, print the usage
information in case of error.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>

scripts/kernel_bump.sh

@@ -205,7 +205,9 @@ main()
 	target_version="${target_version:-${TARGET_VERSION:-}}"
 
 	if [ -z "${source_version:-}" ] || [ -z "${target_version:-}" ]; then
-		e_err "Source (${source_version}) and target (${target_version}) versions need to be defined."
+		e_err "Source (${source_version:-missing source version}) and target (${target_version:-missing target version}) versions need to be defined."
+		echo
+		usage
 		exit 1
 	fi