These callbacks are only triggered either by arguments do not match explicit rquirements of the sepc256k1 library, or by hardware failures, memory corruption
or bug in secp256k1, and not by misuse of the library.
In theory we do not need to implement them, except to find bugs in our own code, but the default callbacks print a message to stderr and call abort() which
is not nice especially on mobile apps.
=> Here we introduce 2 specific exceptions, Secp256k1ErrorCallbackException and Secp256k1IllegalCallbackException, which are thrown when the error callback or illegal callback are called.
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