Issue #1904 Change RAISERROR 'severity' to 16 for those that should throw an error…

[RonMacNeil]

… and error.

Fixes # 1904.

Changes proposed in this pull request:

• Changed the severity value from 0 to 16 for RAISERROR call that should throw an error.

How to test this code:

• Execute this: EXEC sp_BlitzBackups @PushBackupHistoryToListener = 1, @WriteBackupsToListenerName = 'BIG-AG', @WriteBackupsToDatabaseName = 'Backups-AG', @WriteBackupsLastHours = 3000
• It should terminate with an error that looks like this:
  Pushing backup history to listener Msg 50000, Level 16, State 1, Procedure sp_BlitzBackupsR, Line 1187 [Batch Start Line 0] We need a linked server to write data across. Please set one up for BIG-AG.

Has been tested on (remove any that don't apply):

• SQL Server 2016

[BrentOzar]

Great work!