improve error message when pushing with invalid annotation flags

Current behavior and output:

```console
$ oras push --annotation "key:value" ghcr.io/library/alpine:3.9
Error: missing key in `--annotation` flag: key:value
```

Suggested error message:

```console
$ oras push --annotation "key:value" ghcr.io/library/alpine:3.9
Error: annotation value doesn't match the required format.
Try --annotation "key=value"  
```