Add message to set de debugger error handler

[fortizpenaloza]

Add an extension message to WillowApplication in a Development package to set the debugger error handler:

 | application |
 application := MyWillowApplication registerAsDevelopmentApplication.
 application filter configuration
  	  at: #exceptionHandler
  	  put: WADebugErrorHandler.

To a more declarative one, like application beDebuggable or application setDebuggerErrorHandler.

[gcotelli]

I've moved the issue here because it's better to add it as an extension to WAApplication because is not depending on Willow specific code.