dont fatal in Watch.init, return error instead

Fixes ziglang#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

lib/std/Build/Watch.zig

@@ -525,7 +525,10 @@ pub fn init() !Watch {
                 .REPORT_FID = true,
                 .REPORT_TARGET_FID = true,
             }, 0) catch |err| switch (err) {
-                error.UnsupportedFlags => fatal("fanotify_init failed due to old kernel; requires 5.17+", .{}),
+                error.UnsupportedFlags => |e| {
+                    std.log.err("fanotify_init failed due to old kernel; requires 5.17+", .{});
+                    return e;
+                },
                 else => |e| return e,
             };
             return .{