Provide a useful error message when the binding is not found

Currently we show the unhelpful for `libsass` binding not found.
This accounts for a number of reoccurring GitHub issues.
It's also the second largest driver of Google traffic to the
node-sass GitHub repo.

lib/extensions.js

@@ -147,7 +147,11 @@ sass.getBinaryPath = function(throwIfNotExists) {
   }
 
   if (!fs.existsSync(binaryPath) && throwIfNotExists) {
-    throw new Error(['`libsass` bindings not found in ', binaryPath, '. Try reinstalling `node-sass`?'].join(''));
+    throw new Error([
+      ['The `libsass` binding was not found in', binaryPath].join(' '),
+      ['This usually happens because your node version has changed.'],
+      ['Run `npm rebuild node-sass` to build the binding for your current node version.'],
+    ].join('\n'));
   }
 
   return binaryPath;

test/runtime.js

@@ -205,7 +205,7 @@ describe('library detection', function() {
     assert.throws(function() {
       fs.renameSync(originalBin, renamedBin);
       process.sass.getBinaryPath(true);
-    }, /`libsass` bindings not found/);
+    }, /The `libsass` binding was not found/);
 
     fs.renameSync(renamedBin, originalBin);
   });