RFR: 8237967: No proper error message when --runtime-image points to non-existent path

[Andy Herrick]

This simple fix [1] to jpackage bug [2] simply adds a proper error 
message when --runtime-image points to a non-existent directory path.

[1] http://cr.openjdk.java.net/~herrick/8237967/webrev.01/index.html
[2] https://bugs.openjdk.java.net/browse/JDK-8237967

/Andy