Unify Unpacker Error Reporting

[mulhearn]

Just recording this one for now... this won't be in 800.

During IB testing it is clear with two, and soon three, era's to support, we want to move to an unpacker regimen where:

• Every L1T unpacker is always run at RAW2DIGI step
• Every unpacker has a flag "ExpectNonEmptyPayload".
• If ExpectNonEmptyPayload is false, and empty payload encountered, no Error.
• If ExpectNonEmptyPayload is true, and empty payload encountered, issue up to 5 Errors.
• ExpectNonEmptyPayload is only set to true for payloads in critical path
• remove all unpacking from DQM sequences
• ExpectNonEmptyPayload defaults to true, so HLT, which only unpacks required modules, requires no adaptation.

This was always the plan, just, well, you know...