"elastic fail 503::" - Improve log message for better debug

[AndersonQ]

Describe the enhancement:

At least some error logs from fleet server provide little, sometimes almost no information about the root cause of the error.
ErrElastic.Error() method can be improved to account for ErrElastic.Cause not being empty, and produce a more clear message than elastic fail.... elastic is too broad term, it seems to refer to elasticsearch errors, that can be made explicit in the error message as we all documenting the type.

The current Error method often produces elastic fail 503::, which is cryptic for anyone not quite familiar with the code.
Describe a specific use case for the enhancement or feature:

Investigate production issues

[michel-laterman]

This was implemented in https://github.com/elastic/fleet-server/pull/1684/files#diff-fefef321d478decf3f5ced5c3b9e41ad4ebc99b4a8f72cb1a230cd5570161858