Show a better error when MODULE.bazel is missing

Previously, Bazel would error out with this message if it couldn't find
a MODULE.bazel (e.g. in the root module):

```
ERROR: unknown error during computation of main repo mapping
```

Closes #16080.

PiperOrigin-RevId: 466910762
Change-Id: I6fe4fc3e26b5fd203945d7031a9b2a086dce4d31

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleFileFunction.java

@@ -293,7 +293,7 @@ private static byte[] readFile(Path path) throws ModuleFileFunctionException {
     try {
       return FileSystemUtils.readWithKnownFileSize(path, path.getFileSize());
     } catch (IOException e) {
-      throw new ModuleFileFunctionException(e);
+      throw errorf(Code.MODULE_NOT_FOUND, "MODULE.bazel expected but not found at %s", path);
     }
   }