Add last failure info to checks API and CLI command

[benhoyt]

This is a similar but slightly cleaned up version of what we discussed here. We decided against it at the time, but now after having considered using changes/tasks and then events for check diagnostics, we've decided that it's probably better to separate out the events/hooks use case from the debugging use case, and we're back to having a simple way to view the check failures when debugging.

The response from the API now includes a last-failure key as follows (it's only present if failures is greater than 0):

{
  "name": "chk1",
  "status": "down",
  "failures": 1,
  "threshold": 1,
  "last-failure": {
    "error": "an error message",
    "details": "extended details ..."  // this may or may not be present, depending on the failure
  }
}

And the pebble checks CLI command is extended with a Last Failure field and a --verbose option as follows:

# In non-verbose mode (default), the "Last Failure" field and details are included
# on a single line, but truncated to 60 chars max (with "..." suffix if truncated)
$ pebble checks
Check  Level  Status  Failures  Last Failure
chk1   -      down    1/1       an error message: extended details ...

# In --verbose mode, the first line is the error message and subsequent lines show
# the details, if any
$ pebble checks --verbose
Check  Level  Status  Failures  Last Failure
chk1   -      down    1/1       an error message:
                                extended details ...
                                may be multiple lines ...

[benhoyt]

I shortened this check error message slightly as "received" seemed redundant.

[hpidcock]

usually we call them 2xx 3xx 4xx etc when referring to http status codes

[benhoyt]

Good point. Fixed.

[benhoyt]

Per discussion at London mini-sprint, with the new approach of doing checks via changes and tasks properly, we likely don't need this. Though we could show error information pulled from the change/task in pebble checks.

[benhoyt]

Per above, we're planning to do this a different way via changes+tasks.