Wip/h3nk3 watch project files

The improved error message is only useful when there is a template that cannot be found (which happens everytime Activator is started with the Activator root project). Now it is cleared what the error means.

Apart from triggering a ``compile`` upon any source file changes this PR also has some minor refactoring improvements a la the boy scout rule.

The client side of this functionality, i.e. having a flag to toggle compilation on change or not, is __not__ part of this PR. Atm there will always be a recompilation on a file change. 

Further improvements is to check what type of source file it is and take action based on that, e.g. if it is a test file then ``test:compile`` should be invoked instead. As this logic will move into a sbt plugin there has been no such emphasis in this PR. 
this will lock up the UI right now, so maybe we want to at least be sure the default for rerunOnBuild is false ;-)

when run does work, I think in old activator we would kill the previous run in this case before launching a new one. That will be easier once we have the backgroundRun support though so don't worry for now.