Ensure SemaphoreGroupPCD can't be used with STATIC_SIGNATURE_PCD_NULLIFIER

[RiverRuby]

Resolves #133

This PR was initially just meant to tackle issue #133, but in doing that I uncovered some other bugs / incomplete parts of the codebase that were necessary to test if this was done correctly. Here's a total list of what this PR contains:

• Throw an error if user tries to make a SemaphoreGroupPCD with STATIC_SIGNATURE_PCD_NULLIFIER, that notifies them their anonymity would be broken
• Prevent the usePendingPCD hook from continuing to ping the server after receiving a PendingPCD with an ERROR status
• Transport error messages from server to client & display them so that they know what the error was

[ichub]

lgtm after you address the comments.

thanks for fixing up the server-side proving error handling too!