Adds fail-safe for search fileless fetch payloads and updates the docs accordingly

[msutovsky-r7]

The shell-search variant of fileless fetch payloads has been proved to be not fully reliable across all systems as there are systems (dockers, fresh systems with low resources, access as very low-privileged user,..) where the anonymous file handles either don't exist or existing user does not have permission to write the payload into them. This PR mentions this in fetch payload documentation and adds fail-safe mechanism, which defaults to standard fetch payload if no available file handle is found.

[dwelch-r7]

what's the reason for this change here?

[msutovsky-r7]

When the shell-search fails, it should exit the loop and execute the fail-safe fetch command. If there would be only break and shell-search would succeed, it would execute fail-safe regardless. To avoid too much of additional code, the shell-search exits when executed.