i was trying the mercurial plugin and selected repository browser bitbucket but didn't enter an url, this causes an error 500 from tomcat, which make it a bit difficult to figure out what is actually wrong (I though that the repository url was wrong initially).

javax.servlet.ServletException: java.lang.RuntimeException: Failed to instantiate class hudson.plugins.mercurial.MercurialSCM
root cause
java.net.MalformedURLException: no protocol:

the same happens on the config page for cvs, git and svn (at least, thats all scm plugins I have installed), it would be nice if the url is validated on the config page similar to other config parameters to get a red error message.