Add ignore flag if target/classes doesn't exist

[Christian Schulz]

We have configured jdepend in our global parent and it is annoying if the build fails of missing folder in the parent:

[INFO] <<< jdepend-maven-plugin:2.0:generate (default) < compile @ rack-parent <<<
[INFO]
[INFO] --- jdepend-maven-plugin:2.0:generate (default) @ rack-parent ---

Directory does not exist: \rack\target\classes

usage:

JDepend [-components <components>] [-file <output file>] <directory> [directory2 [directory 3] ...]