JDBCError in jdbc/connection.rb misses exception information

JDBCError should say what type of exception was thrown.

In trying to use this module with VoltDB and JRuby,
I got the following error message which really didn't help me.

"ActiveRecord::JDBCError: The driver encountered an unknown error: "
<stacktrace>

Instead, after this change, I got:
"ActiveRecord::JDBCError: The driver encountered an unknown error: java.sql.SQLFeatureNotSupportedException"

which helps me make a more informed decision about my next action