[#28] improves error message when multiple files are found

[EndzeitBegins]

This change improves the message received, when multiple jar files match the given wildcard.
As denoted in #28, previously there only was an unhelpful message by maven, e.g.:

[ERROR] The goal you specified requires a project to execute but there is no POM in this directory (/tmp/build/put). Please verify you invoked Maven from the correct directory. -> [Help 1]

With this change the user is warned clearly that multiple files match the given pattern, e.g.:

  [WARNING] multiple files found:
  ./build-output/your-artifact-1.0.0-rc.0-SNAPSHOT.jar
  ./build-output/your-artifact-1.0.0-rc.0-SNAPSHOT-javadoc.jar
  ./build-output/your-artifact-1.0.0-rc.0-SNAPSHOT-sources.jar
  [INFO] picking file ./build-output/your-artifact-1.0.0-rc.0-SNAPSHOT.jar

@patrickcrocker Would you be so kind and take a look at this, merge if everything looks good to you and publish a new version?

Thank you in advance!