DSAPI should log all responses

[bwmcbride]

During a recent incident, the DSAPI's connection to a data server failed when it was initialising and that endpoint remained unusable. All requests to the endpoint failed, but no log entries were generated for these failures.

This makes it harder for the operators to determine which component is failing and can delay resolution of an issue.

All responses should be logged. In the case of errors due to failed initialisation, there should be some indication in the log entries that this is cause of the failure and a suggestion to look earlier in the log for the cause of the failed initialisation.

See also #49

[bwmcbride]

From @der on https://epimorphics.codebasehq.com/projects/operations/tickets/416

Comment from @bwmcbride on a separate ticket suggests the main logging problem is that there isn't a log message for each failed call to ds-api. That looks to be due to the LogExceptions class letting 500 errors through unlogged (line 38). Looks like an easy fix.