fix(server): Set Server.pendingSnapshot back to nil after saving a snapshot fails.

When a snapshot fails to save, Sever.pendingSnapshot is not set to nil (like it is after a snapshot is successfully saved).  This causes all future calls to TakeSnapshot() to fail with an error "Snapshot: Last snapshot is not finished."

Fixes #1496
Merge pull request #1 from HBOCodeLabs/jstamerj/fixSnapshot

fix(server): Set Server.pendingSnapshot back to nil after saving a snapshot fails.