Improve error message when deploy protocol isn't available

[INFO] --- maven-site-plugin:3.0-SNAPSHOT:deploy (default-deploy) @ asf-pom-site ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.476s
[INFO] Finished at: Sun Jul 24 23:14:10 CEST 2011
[INFO] Final Memory: 19M/342M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-site-plugin:3.0-SNAPSHOT:deploy (default-deploy) on project asf-pom-site: Unsupported protocol: 'scp': Cannot find wagon which supports the requested protocol: scp: java.util.NoSuchElementException
[ERROR] role: org.apache.maven.wagon.Wagon
[ERROR] roleHint: scp
[ERROR] -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException