Better error message when java home is missing src dirs

[devinrsmith]

During the install process, jpy may error out with

Error: environment variable "JAVA_HOME" must be set to a JDK (>= v1.7) installation directory

This is a confusing error message when JAVA_HOME is set, see #106. In this case, the "lib" source directory was missing. It would probably be prudent to error out with a more explicit error message when one of the java home environment variables is set and either the directory doesn't exist, or the directory exists but the needed subdirectories don't exist.