0.9.1 to fix missing Error import.
0.9.1 to fix missing Error import.

Fixes #187.

jmwri/voluptuous,alecthomas/voluptuous,tuukkamustonen/voluptuous,SimplyKnownAsG/voluptuous,SimplyKnownAsG/voluptuous,divanovGH/voluptuous,tuukkamustonen/voluptuous,alecthomas/voluptuous