Stop returning `NOTCAPABLE` errors from WASI calls.

[sunfishcode]

ENOTCAPABLE was an error code that is used as part of the rights system, from CloudABI. There is a set of flags associated with each file descriptor listing which operations can be performed with the file descriptor, and if an attempt is made to perform an operation with a file descriptor that isn't permitted by its rights flags, it fails with ENOTCAPABLE.

WASI is removing the rights system. For example, WebAssembly/wasi-libc#294 removed support for translating ENOTCAPABLE into POSIX error codes, on the assumption that engines should stop using it.

So as another step to migrating away from the rights system, remove uses of the ENOTCAPABLE error.

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[sunfishcode]

Good call, I applied both your suggestions :smile:.