fuzz: Fix RPC internal bug detection #23707

Previously the fuzz test considered any exception which contains the string Internal bug detected (magic string) as a bug. This is not true when the user (fuzzer) passes in the magic string from outside.

Fix that by:

1. Changing the format the string in NonFatalCheckError to start with the magic string.
2. Only treat exceptions that start with the magic string as internal bugs.

This should fix the bug because any other exception shouldn't start with the magic string.

To test:

echo 'bG9nZ2luZ1y+bUludGVybmFsIGJ1ZyBkZXRlY3RlZAAXCqNcjqNcjuYjeg==' | base64 --decode > /tmp/a
FUZZ=rpc ./src/test/fuzz/fuzz /tmp/a

Before:

fuzz: test/fuzz/rpc.cpp:365: void rpc_fuzz_target(FuzzBufferType): Assertion `error_msg.find("trigger_internal_bug") != std::string::npos' failed.

After:

Executed /tmp/a in 0 ms

crACK fa77f95c2ff4ae7761208d06bcbeb59650612367