Add a SM call that allows for an enclave to exit and mark itself as "failed"

[chungmcl]

From discussion with @dkohlbre

We should have a way for an enclave to exit/stop and specify that execution of the runtime failed and that this enclave should be not be run again. Currently the SM's exit/stop are very similar in functionality and neither allow the enclave to specify it failed.

[dkohlbre]

After talking with Dayeol, this was the intended purpose of the exit SBI call. We will want to update it to actually mark the enclave as non-resumeable.
This will also require updating the docs and figures to indicate exit does this.