dont fatal in Watch.init, return error instead

[mocompute]

Fixes #21543.

Since UnsupportedFlags was caught explicitly, I replaced the fatal() call with a std.log.err to maintain that behaviour. Returning an error from init() will help clients fail gracefully, especially zls: zigtools/zls#2041

[mocompute]

In this repository, the only call to Watch.init(), which now returns an error instead of a fatal process exit, is from build_runner.zig's main function, so returning an error will have the same effect of exiting the process. But this change will allow zls to handle the failure gracefully,

zig/lib/compiler/build_runner.zig

Line 401 in 3b465eb

var w = if (watch) try Watch.init() else undefined;