Provide options to raise Exception while there is no snapshot exists during testing

[lucemia]

Currently snapshottest won't raise Exception while there is no snapshot exists. It is not expect behavior during test.
It would be useful if there is an options or flags to let snapshottest raise exception in such case.

VCR's has a similar options call once provide this functions.
https://vcrpy.readthedocs.io/en/latest/usage.html#once