Notify user on attempt to load an invalid package

Use the log to provide feedback to the user when attempting to load
packages that are not correctly configured. This is mainly of benefit
when using the interface at index.php/dashboard/extend/install to
install new packages:

- If your Package class sets $pkgHandle to a value other than
  the package directory, concrete5 isn't able to load the package when
  you click "install". Previously this was a silent failure - nothing
  happened when you clicked the install button, the package just
  remained in the "Awaiting Installation list". Now the user is notified.

- If a Package class name doesn't follow the expected convention, the
  package won't appear at all in the "Awaiting Installation" list. In
  this situation, the user now has access to some debug information.