fs: throw errors from sync branches instead of separate implementations

Previously to throw errors from C++ land, sync versions of the fs were created by copying C++ code from the original implementation and moving JS code to a separate file. This can lead to several problems:

1. By moving code to a new file for the sake of moving, it would be harder to use git blame to trace changes and harder to backport changes to older branches.
2. Scattering the async and sync versions of fs methods in different files makes it harder to keep them in sync and share code in the prologues and epilogues.
3. Having two copies of code doing almost the same thing results in duplication and can be prone to out-of-sync problems when the prologue and epilogue get updated.
4. There is a minor cost to startup in adding an additional file. This can add up even with the help of snapshots.

This patch moves the JS code back to lib/fs.js to stop 1, 2 & 4 and introduces C++ helpers SyncCallAndThrowIf() and SyncCallAndThrowOnError() so that the original implementations can be easily tweaked to allow throwing from C++ and stop 3.