Bug box in case of Program_Error and related tweaks

[mosteo]

Instead of the terse please re-run..., say something more informative and conducive to a report, but only for true Program_Error exceptions:

$ alr dev --raise
error: ******************* BEGIN Alire bug detected *******************
error: Location  : Alr.Commands.Dev.Execute at alr-commands-dev.adb:77
error: Extra info: Raising forcibly
error: Report at : https://github.com/alire-project/alire/issues/new?title=[Bug%20box]
error: Re-run with `-vv -d` for a full stack trace.
error: ******************** END Alire bug detected ********************

In very limited situations where we can continue, report instead a recoverable error:

$ alr dev --error
warn: ******************* BEGIN Alire bug detected *******************
warn: Location  : Alr.Commands.Dev.Execute at alr-commands-dev.adb:73
warn: Extra info: Forced error
warn: Report at : https://github.com/alire-project/alire/issues/new?title=[Bug%20box]
warn: Re-run with `-vv -d` for a full stack trace.
warn: Alire will now continue as the error might be recoverable.
warn: ******************** END Alire bug detected ********************

And finally clarify internally the difference with a Recoverable_User_Error.

[Fabien-Chouteau]

adequate?

[mosteo]

My Spanglish seeping through