Cherry-pick #18405 to 7.x: Cleaner output of inspect command

[michalpristas]

Cherry-pick of PR #18405 to 7.x branch. Original message:

What does this PR do?

Use of classic errors instead of custom type for inspect command. custom type is more suitable for logs where it carries call stack and error history here we just need an output displayed to user.

Why is it important?

call stack is confusing for user

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test

build agent and provide config with management.mode=fleet without enrolling. then run ./elastic-agent inspect
You should see error without call stack

Fixes: #18344

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request Cherry-pick #18405 to 7.x: Cleaner output of inspect command  #18560 updated]

• Start Time: 2020-05-15T13:18:36.294+0000

• Duration: 76 min 55 sec (4555286)

Test stats 🧪

Test	Results
Failed	0
Passed	1820
Skipped	182
Total	2002

[blakerouse]

Backport looks good.