Give better error message on Clevis unlock failure

[jbaublitz]

This PR fixes a bug in our clevis decrypt command where it reported success in failure cases and also provides better error messages to the user when the failure occurs.

cc @martinpitt

[jbaublitz]

Example error message:

Execution failed:
stratisd failed to perform the operation that you requested. It returned the following information via the D-Bus: ERROR: An attempt to set up pool with UUID e94a3e25-229a-4752-8715-c97cae0fdd75 from the assembled devices failed; Failed to unlock using token; Failed to retrieve Clevis passphrase; Error communicating with server localhost. 

[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo yum install -y dnf-plugins-core on RHEL 8
• sudo dnf install -y dnf-plugins-core on Fedora
• dnf copr enable packit/stratis-storage-stratisd-3741
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.

[martinpitt]

Thanks @jbaublitz!