Don't capitalize error string in union serializer/deserializer (#68)
Don't capitalize error string in union serializer/deserializer (#68)

...for generated code.

See
https://github.com/golang/go/wiki/CodeReviewComments#error-strings
and
http://staticcheck.io/docs/checks#ST1005
alanctgardner/gogen-avro,alanctgardner/gogen-avro