Fix panic in mistralrs-server

[cdoko]

The completion.rs file appears to have been initially based on the chat_completion.rs file, but the match arms were not fully adapted for the completion use case, resulting in a missing CompletionModelError and an unnecessary ModelError case.

This PR also sets self.is_done to true for CompletionModelError and ModelError cases, which ends the stream after an error, similar to the behavior for Chunk/CompletionChunk arms when finish_reason is Some.

[github-actions]

Code Metrics Report

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 Language            Files        Lines         Code     Comments       Blanks
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 C Header                2           35           28            0            7
 Dockerfile              1           41           22           10            9
 JSON                   12          105          104            0            1
 Python                 64         2729         2359           71          299
 Shell                   1           57           22           18           17
 Plain Text              3         3723            0         2413         1310
 TOML                   18          611          544            2           65
 YAML                    2           21           19            2            0
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 Jupyter Notebooks       4            0            0            0            0
 |- Markdown             2           77           32           31           14
 |- Python               2          205          178            1           26
 (Total)                            282          210           32           40
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 Markdown               44         3460            0         2625          835
 |- BASH                 6          103          100            0            3
 |- JSON                 1           12           12            0            0
 |- Python               7          121          109            0           12
 |- Rust                13          440          373            0           67
 |- TOML                 2           75           63            0           12
 (Total)                           4211          657         2625          929
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 Rust                  299        94302        84614         1921         7767
 |- Markdown           145         1617           25         1469          123
 (Total)                          95919        84639         3390         7890
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 Total                 450       105084        87712         7062        10310
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