Report some types of errors in start-proxy status reports

[mbg]

This PR allows the start-proxy action to report some types of errors in status reports. The basic idea here is that we have a StartProxyErrorType enum to represent types of failures we care about, with associated, fixed error messages. We then have a StartProxyError type for exceptions which only accepts members of the StartProxyErrorType enum as messages.

Most of the size of this PR comes from refactoring functions from start-proxy-action to start-proxy to make it easier to handle the individual error cases, and to add better unit test coverage.

Best reviewed commit-by-commit.

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