Better Slave Decommissioned / Offline Communication

[Calvinp]

• The alert for a slave decommissioning is now at warn level, not danger level
• The wording of the alert now suggests that this is a normal Singularity action, nothing is on fire, your task has been replaced, and you can continue about your day without (much) worry
• If a slave is offline, the scary ajax error stating so is now caught if it appears when trying to fetch files
• Instead of the scary ajax error, the files table now says that the slave is offline so we don't have files.

[ssalinas]

Thanks for this @Calvinp , messaging is much clearer (and less alarming!) now