Improve "No project root found" error message

This error message is too vague and can be confusing for folks. We can
make it better by providing some more context.

More info: Galooshi/atom-import-js#13

lib/__tests__/findProjectRoot-test.js

@@ -25,7 +25,7 @@ it('treats package folders as roots', () => {
 
 it('throws if it can not find a folder', () => {
   expect(() => findProjectRoot('/foo/bar/baz.js')).toThrowError(
-    'No project root found',
+    'No project root found, looking for a directory with either a package.json file or a node_modules directory.',
   );
 });
 

lib/findProjectRoot.js

@@ -5,7 +5,7 @@ const NODE_MODULES_PATTERN = /\/node_modules$/;
 
 function findRecursive(directory) {
   if (directory === '/') {
-    throw new Error('No project root found');
+    throw new Error('No project root found, looking for a directory with either a package.json file or a node_modules directory.');
   }
   const pathToPackageJson = path.join(directory, 'package.json');
   const pathToNodeModulesFolder = path.join(directory, 'node_modules');