be more explicit with problems using coreDir option

the coreDir parameter might also be invalid if a file in coreDir is not found.
Re-phrase the warning to also match this possibility
completely rely on coreDir for dependencies

bootstrap, bootstrap-datepicker and font-awesome should be shipped with the
core. In order to prevent versioning conflicts, we rely on those instead of
local versions from bower_components/.
simplify "valid coreDir" detection

issue a warning and don't compile any less if no apps/themes/style.less is
found.