Use more descriptive error messages in model proxy

[kian99]

Description

This PR fixes an issue in tests that were checking the error from ProxySockets.

The model proxy returns an error once the proxy is completed but there are actually 2 errors emitted, 1 from the client side connection and 1 from the controller side connection, because only the first of the two errors is returned there is a sort of race in which message the calling function sees.

We fix this by using more descriptive error messages in the model proxy and then using an appropriate regex in tests.

Debugging the race condition was made easy by running the tests with a high count, so running the following:
go test -timeout 30s -run ^TestProxySocketsAuditLogs$ github.com/canonical/jimm/v3/internal/rpc -count=1000
surfaced the issue consistently

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

[ale8k]

abnormal closures aren't normal, this just masks an actual issue

[kian99]

abnormal closures aren't normal, this just masks an actual issue

@ale8k I think that's a separate matter we can look into and address. For this PR giving the different errors that come out of the model proxy their own unique messages highlights the problem and the tests have been accordingly. Let's fix any issues with abnormal closure messages separately.

[ale8k]

Approving but plz plz can we fix abnormal closures asap? If there are any issues?