improve error message when pushing a manifest without media type #1212
Description
Current behavior and output:
$ oras manifest push --oci-layout /sample/images:foobar:mediatype
Error: media type is not recognized. Suggested error message:
$ oras manifest push --oci-layout /sample/images:foobar:mediatype manifest.json
Error: media type is not specified via the flag "--media-type" nor in the manifest.json
Usage: oras manifest push [flags] <name>[:<tag>[,<tag>][...]|@<digest>] <file>
You need to specify a valid media type in the manifest JSON or via the "--media-type" flag