1030604: Single error message when adding blacklisted overrides

No longer do a toString on the collection of messages. Instead,
build a list of non-existant repo names, and formulate a single
error message.
Use isEmpty instead of checking size
Fixed issue with checking override black list

Index 0 was not being included in the black list check.
Bumping rules minor version
Merge pull request #439 from candlepin/mstead/single-add-override-message

Single error message when adding blacklisted overrides