Improve error logs when flag evaluation failed

[Kavindu-Dodan]

Current SDK emits following log line when a flag evaluation fails,

Unable to correctly evaluate flag with key <flag_key> due to exception <exception_message>

Ex:- from Java getting started guide when flagd is not up and running

This error hides the exception stack trace, making it difficult to debug errors.

Proposal

Include the stack trace for better error details