Prepend a slash to a path if it does not start with one. Fixes opening
rascal:// locs from stacktrace in editor.
Merge branch 'master' of https://github.com/cwi-swat/rascal.git
Merge branch 'master' of https://github.com/cwi-swat/rascal.git
Merge pull request #326 from Y3PP3R/master

Opening rascal:// locs on windows in stacktrace