Improve error message for "SHA1 not computed"

Summary:
This error message isn't accurate since Metro added symlink support, and doesn't suggest the most likely cause, which is that the requested file isn't watched.

Metro's resolver should never resolve an unwatched file, but custom resolvers (like `rnx-kit`'s) might.

Changelog: Internal

Reviewed By: vzaidman

Differential Revision: D69397742

fbshipit-source-id: 2fd141387d3201ed321d644c80618b3bc12052b5

packages/metro/src/node-haste/DependencyGraph.js

@@ -261,10 +261,12 @@ class DependencyGraph extends EventEmitter {
 
     if (!sha1) {
       throw new ReferenceError(
-        `SHA-1 for file ${filename} is not computed.
+        `Failed to get the SHA-1 for ${filename}.
          Potential causes:
-           1) You have symlinks in your project - watchman does not follow symlinks.
-           2) Check \`blockList\` in your metro.config.js and make sure it isn't excluding the file path.`,
+           1) The file is not watched. Ensure it is under the configured \`projectRoot\` or \`watchFolders\`.
+           2) Check \`blockList\` in your metro.config.js and make sure it isn't excluding the file path.
+           3) The file may have been deleted since it was resolved - try refreshing your app.
+           4) Otherwise, this is a bug in Metro or the configured resolver - please report it.`,
       );
     }