Use Throwable.initCause() to improve error reporting

As Derby has moved to J2SE 1.4, Throwable.initCause() can now be used to improve error reporting.
As stated in the initial commit (that was backed out, see below):
'Set Throwable.initCause() in a couple of locations at the JDBC level where we setup nested SQLExceptions.
This means that automatically the stack trace for a database failed to start includes the reason
for the failure, rather than just "see next exception". This is a great help when running
JUnit tests and getting failures to start a database.'

The initial commit was backed out because it broke numerous tests under JDK 1.6, and the author indicated he had no time to investigate.
The patch caused no failures when running the tests with Java SE 5.0.