spirv-dis: Don't break on unrecognized capability

dpcpp is generating modules with its own caps. spirv-dis reacts like this:
```
OpCapability Addresses
OpCapability Linkage
OpCapability Kernel
OpCapability Int64
OpCapability GenericPointer
OpCapability Int8
error: 7: Invalid capability operand: 5629
```

Can the tool just print the numeric operand of the OpCapability and continue? The instructions are length prefixed so there's no reason to break. Ditto with spirv-val.

Start by tracing how spirv-dis handles an unrecognized OpCapability operand, then inspect the corresponding spirv-val path. Use the shown numeric capability case to verify that both tools print the operand and continue parsing the length-prefixed instruction instead of stopping.

Written by the indexing model from the issue text.