Allow elastic-agent diagnostics collect to ignore failures

[michel-laterman]

Describe the enhancement:

The elastic-agent diagnostics collect command will not produce an archive if it encounters a failure gathering logs or config data (and possible meta-data).
The collect command should be able to make a note of these errors in the archive (and echo them to stderr) and continue attempting to gather more data.

Describe a specific use case for the enhancement or feature:

When gathering diagnostics information it is preferable to gather an incomplete archive then no archive.

[michel-laterman]

cc @ruflin @ph @jlind23

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[michel-laterman]

As reported in elastic/beats#30059, metadata collection will stop gathering data from processes if one fails.

The behaviour should be to output the error and continue collecting from the other processes.