Tweak error message and make TransportException extend from RuntimeException, refs #2753

src/Composer/Downloader/DownloadManager.php

@@ -191,6 +191,9 @@ class DownloadManager
         $this->filesystem->ensureDirectoryExists($targetDir);
 
         foreach ($sources as $i => $source) {
+            if (isset($e)) {
+                $this->io->write('<warning>Now trying to download from ' . $source . '</warning>');
+            }
             $package->setInstallationSource($source);
             try {
                 $downloader = $this->getDownloaderForInstalledPackage($package);
@@ -204,9 +207,9 @@ class DownloadManager
                 }
 
                 $this->io->write(
-                    '<warning>Caught an exception while trying to download '.
-                    $package->getPrettyString().
-                    ': '.
+                    '<warning>Failed to download '.
+                    $package->getPrettyName().
+                    ' from ' . $source . ': '.
                     $e->getMessage().'</warning>'
                 );
             }

src/Composer/Downloader/TransportException.php

@@ -15,7 +15,7 @@ namespace Composer\Downloader;
 /**
  * @author Jordi Boggiano <j.boggiano@seld.be>
  */
-class TransportException extends \Exception
+class TransportException extends \RuntimeException
 {
     protected $headers;