Fix very long job failure text
# What does this PR do?

Slack SDK have a limit of `3000` characters for "text" field. For some rare cases, we have very long test error trace.
See this [dummy job run](https://github.com/huggingface/transformers/runs/6753872641?check_suite_focus=true)

This PR tries to limit the length, so the report could be sent to Slack.
(Previous version has some check, but it didn't take into account that a single test failure could have very long trace itself along)


For the record, the test failure with very long trace is like
```
E           ValueError: torch.float32 is enabled but the following parameters have dtype that is not torch.float32: [('shared.weight', torch.float16), ('encoder.block.0.layer.0.SelfAttention.q.weight', torch.float16), ('encoder.block.0.layer.0.SelfAttention.k.weight', torch.float16), ('encoder.block.0.layer.0.SelfAttention.v.weight', torch.float16), ('encoder.block.0.layer.0.SelfAttention.o.weight', torch.float16), ('encoder.block.0.layer.0.SelfAttention.relative_attention_bias.weight', torch.float16), ('encoder.block.0.layer.0.layer_norm.weight', torch.float16), ('encoder.block.0.layer.1.DenseReluDense.wi.weight', torch.float16), ('encoder.block.0.layer.1.DenseReluDense.wo.weight', torch.float16), ('encoder.block.0.layer.1.layer_norm.weight', torch.float16), ('encoder.block.1.layer.0.SelfAttention.q.weight', torch.float16), ('encoder.block.1.layer.0.SelfAttention.k.weight', torch.float16), ('encoder.block.1.layer.0.SelfAttention.v.weight', torch.float16), ('encoder.block.1.layer.0.SelfAttention.o.weight', torch.float16), ('encoder.block.1.layer.0.layer_norm.weight', torch.float16), ('encoder.block.1.layer.1.DenseReluDense.wi.weight', torch.float16), ('encoder.block.1.layer.1.DenseReluDense.wo.weight', torch.float16), ('encoder.block.1.layer.1.layer_norm.weight', torch.float16), ('encoder.final_layer_norm.weight', torch.float16), ('decoder.block.0.layer.0.SelfAttention.q.weight', torch.float16), ('decoder.block.0.layer.0.SelfAttention.k.weight', torch.float16), ('decoder.block.0.layer.0.SelfAttention.v.weight', torch.float16), ('decoder.block.0.layer.0.SelfAttention.o.weight', torch.float16), ('decoder.block.0.layer.0.SelfAttention.relative_attention_bias.weight', torch.float16), ('decoder.block.0.layer.0.layer_norm.weight', torch.float16), ('decoder.block.0.layer.1.EncDecAttention.q.weight', torch.float16), ('decoder.block.0.layer.1.EncDecAttention.k.weight', torch.float16), ('decoder.block.0.layer.1.EncDecAttention.v.weight', torch.float16), ('decoder.block.0.layer.1.EncDecAttention.o.weight', torch.float16), ('decoder.block.0.layer.1.layer_norm.weight', torch.float16), ('decoder.block.0.layer.2.DenseReluDense.wi.weight', torch.float16), ('decoder.block.0.layer.2.DenseReluDense.wo.weight', torch.float16), ('decoder.block.0.layer.2.layer_norm.weight', torch.float16), ('decoder.block.1.layer.0.SelfAttention.q.weight', torch.float16), ('decoder.block.1.layer.0.SelfAttention.k.weight', torch.float16), ('decoder.block.1.layer.0.SelfAttention.v.weight', torch.float16), ('decoder.block.1.layer.0.SelfAttention.o.weight', torch.float16), ('decoder.block.1.layer.0.layer_norm.weight', torch.float16), ('decoder.block.1.layer.1.EncDecAttention.q.weight', torch.float16), ('decoder.block.1.layer.1.EncDecAttention.k.weight', torch.float16), ('decoder.block.1.layer.1.EncDecAttention.v.weight', torch.float16), ('decoder.block.1.layer.1.EncDecAttention.o.weight', torch.float16), ('decoder.block.1.layer.1.layer_norm.weight', torch.float16), ('decoder.block.1.layer.2.DenseReluDense.wi.weight', torch.float16), ('decoder.block.1.layer.2.DenseReluDense.wo.weight', torch.float16), ('decoder.block.1.layer.2.layer_norm.weight', torch.float16), ('decoder.final_layer_norm.weight', torch.float16)]
```

_The documentation is not available anymore as the PR was closed or merged._<|||||>FMI:

Not urgent, but we also have to deal with other blocks, considering the failure below
https://github.com/huggingface/transformers/actions/runs/2472383354

(The summary might be too long when a commit introduces too many test failures)

I will work on it at some point