Add more output when Agent can't install Endpoint

[ferullo]

When Agent fails to install Endpoint the result shown in the Agent log is:

State changed to FAILED: operation ‘Exec’ failed:

which doesn't provide much details to a user. Can we work together to improve that output?

When Endpoint is installed the command line option --log stdout can print Endpoint install logs. Ideally we'd have a way to share those logs with a user but we can explore other options if that's not feasible.

It's worth noting, if Endpoint is not signed (i.e. if it is an internal development build) then some Windows variants will not even run the Endpoint installer. Agent can write code to diagnose and inform a user when that's the case but since that is not an issue that should affect customers I wouldn't consider it high priority or necessary.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)