Update error_messages to Base 1.8.1

The Base message registry has updated to 1.8.1.  This updates our
error_messages files to match.

This changes from the deprecated 'Severity' to the new
'MessageSeverity' property.

It also adds a script to compare our error_messages.cpp messages
against the Base message registry and flag any differences.

Tested:
Ran the Redfish Validator and confirmed that this change does
not introduce any new failures.

Change-Id: I2e5101a5b4d0c0963569493451f99521e42b0f4d
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>