verify-blob: make the signature flag mandatory

[shibumi]

Summary

Before this commit, the verify-blob command would fail ungracefully in experimental mode
if no signature has been provided. This commit adds a better fitting error message.

Before this change:

❯ COSIGN_EXPERIMENTAL=1  cosign verify-blob /tmp/hello-world.txt
Error: verifying blob [/tmp/hello-world.txt]: read .: is a directory
main.go:46: error during command execution: verifying blob [/tmp/hello-world.txt]: read .: is a directory

After this change:

❯ COSIGN_EXPERIMENTAL=1  cosign verify-blob /tmp/hello-world.txt
Error: verifying blob [/tmp/hello-world.txt]: missing flag '--signature'
main.go:46: error during command execution: verifying blob [/tmp/hello-world.txt]: missing flag '--signature'

Ticket Link

None

Release Note

Fixed: verify-blob error message for missing signature flag