Support a public API that allows applications to set up the error logging callback.

For debugging and tracking any errors inside forestdb, we need to provide the new API for registering the error logging callback:

fdb_status fdb_set_log(logCallbackFunc, void *pCtxData);

logCallbackFunc passed by applications can be something like:

void logCallbackFunc(int err_code, const char *err_msg, void *pCtxData)

{ fprintf(stderr, "error: %d, '%s'\n", err_code, err_msg); }

pCtxData is used for passing any application-specific context data to the logging callback.