improve error message when JAVA_HOME is set to the wrong value

This is small (Windows-based) developer usability improvement.

When the JAVA_HOME environment variable is set, but the value is wrong (so that JAVA_HOME + /bin/java.exe does not point correctly to java.exe), the startup script will simply fail with the message

Error: JAVA_HOME is incorrectly set.

which is a bummer. 😞

With this tiny change, the error message will be much friendlier:

Error: JAVA_HOME is incorrectly set: C:\Program Files\Java\jre1.8.0_201\bin
Expected to find java.exe here: C:\Program Files\Java\jre1.8.0_201\bin\bin\java.exe

(in this case showing a situation where one has inadvertently included /bin in the JAVA_HOME environment variable).

This will also give a nicer error message in situations, where the JRE has been updated, and the one pointed to by JAVA_HOME has been uninstalled.