WasmFS: Error handling in OPFS backend

[tlively]

This issue tracks the remaining backend APIs that need proper error handling in the OPFS backend or in the core system.

• Directory::insertDirectory ([WasmFS] Report an optional error from insertDirectory #17771)
• Directory::insertMove ([WasmFS] Allow move to return specific error codes #17785)
• Directory::removeChild ([WasmFS] Allow removeChild to return specific error codes, and use it in OPFS #17793)
• Directory::getEntries ([WasmFS] Return errors from getEntries and getNumEntries #17807)
• Directory::getNumEntries ([WasmFS] Return errors from getEntries and getNumEntries #17807)
• DataFile::getSize ([WasmFS] Report errors from getSize #17783)
• DataFile::flush ([WasmFS] Allow backends to report errors from flush #17781)