Notify failure to create a process when the executable is not found even in PATH.

std/os/child_process.zig

@@ -610,6 +610,9 @@ pub const ChildProcess = struct {
                 } else {
                     return err;
                 }
+            } else {
+                // Every other error would have been returned earlier.
+                return error.FileNotFound;
             }
         };