Improve the messaging of fleetctl debug errors

[roperzh]

Goal

Improve the clarity of the output of fleetctl debug errors. The current output of the command is:

~/projects/fleet $ ./build/fleetctl debug errors
Output written to fleet-errors-2022-05-02T14:39:24-03:00

How?

• Add a warning prompting the user to clean up any sensitive data from the file before sharing
• Make the output more visible
• add a flag to print the output to stdout
• (optional) add the json extension to the generated file

[roperzh]

we should consider adding the warning about sensitive data to the fleetctl debug archive command too.