Improve navigator.share example

Better error handling, was failing silently when not supported.

New version is similar to code example at https://developer.mozilla.org/en-US/docs/Web/API/Navigator/canShare#examples

web-share/index.html

@@ -22,18 +22,26 @@ <h1>Sharing MDN</h1>
         title: 'MDN',
         text: 'Learn web development on MDN!',
         url: 'https://developer.mozilla.org',
-      }
+      };
 
       const btn = document.querySelector('button');
       const resultPara = document.querySelector('.result');
 
       btn.addEventListener('click', () => {
+        if (!navigator.canShare) {
+          resultPara.textContent = 'Web Share API not available';
+          return;
+        }
+        if (!navigator.canShare(shareData)) {
+          resultPara.textContent = 'Share data unsupported, disallowed, or invalid';
+          return;
+        }
         navigator.share(shareData)
           .then(() =>
-            resultPara.textContent = 'MDN shared successfully'
+            resultPara.textContent = 'MDN shared successfully';
           )
           .catch((e) =>
-            resultPara.textContent = 'Error: ' + e
+            resultPara.textContent = 'Error: ' + e;
           )
       });
     </script>