Better Error Reporting

[gmatht]

Please describe the feature you want
In many cases Tabby should be able to report what is wrong on the server console (perhaps only if -v is on the commandline), in the content of the HTTP error response, on the Web UI and/or in a .log file inside ~/.tabby.

Additional context
For example, If we do not run tabby serve with --model the Tabby server will start up without any error messages but reject all requests with a 501 Error. I did not find any explaination for this in any output or log files. It could have reported an error message like "No --model parameter was specified on the Tabby server command line. Re-run Tabby with a --model parameter to fulfill completion requests".

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Please reply with a 👍 if you want this feature.

[wsxiaoys]

related #3239