Handle and log error when a DB instance cannot be found

[schleyfox]

Not a huge issue, but this patch prevents some of our rapidly created or destroyed instances from gumming up the works.

/cc @jtai @ktrice

[ktrice]

good call. looks good

[pcarrier]

👍

[pcarrier]

So the traditional way to log a message with an exception with Slf4j is:

log.warn("message", exception);

That way the logger will, depending on configuration, provide a stacktrace (usually compacted), and/or the exception message, and/or the exception class (usually all of those, and it will do that as it walks up the causality chain so nicely provided by Java exceptions).

Instead of building your string with + (which javac will internally convert to invocations of a StringBuilder constructor if few enough, a constructor then a few appends otherwise), it's best to use the logger's string formatting support. The idea there is that if a log message won't be printed anyway (eg because the log level is disabled), it'll avoid unnecessary work. Stupid example:

log.warn("foo {} baz", "bar", exception);

Essentially, {} in the message format will consume parameters, and the last parameter that's an exception shouldn't be consumed by the message format as the logger will deal with it as it sees fit.