Improved error messages on write thread failure

[C0urante]

When a task fails, the error message can be vague and unhelpful in a couple of ways:

• The construction of the error string in KCBQThreadPoolExecutor::maybeThrowEncounteredErrors throws away the actual stack trace of the encountered error(s), which for things like null pointer exceptions is essential for being able to quickly identify the cause of the failure
• The infamous "Attempted to reduce batch size below 1." error contains no user-actionable information and sheds very little light on what may be going wrong (usually, this error arises when a non-retriable error has been misclassified as a retriable one, which should hopefully become less likely with GH-144: Isolate error classification logic into single, testable class #145)

We should improve the failure messages for the connector in these cases.

[C0urante]

Addressed by #150