Update invalid_name msg in PB index create

Recently there was a change to only allow ASCII chars in the index name.
This updates the PB interface error message when an index name is
invalid.  The HTTP resource was already updated in the original commit
bb44381.
Merge pull request #390 from basho/bugfix/update-pb-msg

Update invalid_name msg in PB index create

Reviewed-by: coderoshi