uSync: surface real error on template create failure

Repo: KevinJump/uSync

Fixed #1044: template import failures reported a hardcoded "Failed to create template" with a null exception, hiding the real cause — especially bad on IIS/Production where it only reproduced in that environment
TemplateSerializer now surfaces Umbraco's real TemplateOperationStatus in the message and wraps it in an exception, matching the "don't discard save-attempt results" fix already applied elsewhere in #1039/#1041
Removed a dead/unreachable duplicate null check left over from an earlier refactor
Added test coverage for the create failure and success paths