Allow send_error to continue if collecting multiple error messages before throwing

[nealkruis]

Thoughts:

• send_error should throw by default, but have an optional argument to force the execution to continue without throwing
• the courier instance should keep track of the number of unthrown errors it has issued
• add a new method for users to indicate they are ready to throw after collecting potential errors (maybe check_for_unthrown_errors?)
• When a courier is destroyed, it should check if there are unthrown errors and do something appropriate