Rework exceptions

[mahesh-hegde]

• Currently an exception raised in JNI, when caught by utility methods in jni_object.dart, does not print the java stack trace. Ideally it should provide a method to print stack trace to stderr.

• We need to raise a detailed exception when a double-free is attempted on a wrapped JNI reference, with details of object and the JNI pointer.

• Currently I am throwing raw strings as exceptions, create few exception classes for more clarity.

Grouping these under same issue and assigning to myself.

[mahesh-hegde]

• Maybe: Lookup exception class, and keep a reference to toString, rather than looking up toString method every time.