[system/process] add support for mutlierr

[VihasMakwana]

What does this PR do?

• Previously, we weren't passing errors to the caller while monitoring set of processes.
  • With the recent introduction of the status reporter for metricsets, it is impossible to change the status to degraded if such errors are not passed to the caller.
• Fix this by passing errors to the caller. We also populate the process related information to our best-effort.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added an entry in CHANGELOG.md

Manual testing and general information

• See https://github.com/elastic/beats/pull/40400 for testing it on metricbeat NOTE:
  • Only applicable if you're using system/process module
  • Non-fatal errors are only received when you have insufficient privileges.

Steps:

• While receiving any error, test for nature of error
• call errors.Is(err, NonFatalErr{})) on received error
  • If true, error is non-fatal and you can proceed further (metrics will be partially available, most probably insufficient privileges).
  • Else, log the error and stop execution (metrics will be empty)

Genreal info related to the changes in this PR:

• While getting process related information, you might also receive a non-nil error.
• Such errors come in two flavours:
  • Fatal errors:
    • This indicates that the error was fatal (for eg. no process found,)
    • Caller should stop further execution if they receive fatal errors
  • Non-fatal errors:
    • This indicates that the error was non-fatal (for eg. not enough privileges).
    • It means that metrics are partially filled.
    • Further execution can be continued if non-fatal errors are encountered

• Closes https://github.com/elastic/elastic-agent-system-metrics/issues/164

[VihasMakwana]

@rdner I've added the beats PR link in description