server : fix crash when using verbose output with input tokens that are not in printable range (#12178)

[ishaangandhi]

This PR fixes #12178.

Previously, when calling /completions with a token outside of the token vocabulary range, you would get this exception:

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 18446744073709551615) >= this->size() (which is 151936)

Now, the server continues to run, and the user gets a useful error:

{"error":{"code":400,"message":"Prompt contains invalid tokens","type":"invalid_request_error"}}%                     

[ngxson]

Bonus: would be nice if you can add a test case for it, see server/tests/test_tokenize.py

[ishaangandhi]

@ngxson are you OK to merge this? I rebased the changes.

re: adding a test - I think test_tokenize tests the /tokenize and /detokenize endpoints, which invoke a different path than what we fix in this PR.

How about we merge this as-is?

[ngxson]

I only refer to that file so you can see how the code looks like. Adding test should be easy and it is a good way to show that your code actually work.