Report errors when a git repo can not be synced up

src/Composer/Repository/Vcs/GitDriver.php

@@ -40,7 +40,9 @@ class GitDriver extends VcsDriver
 
             // update the repo if it is a valid git repository
             if (is_dir($this->repoDir) && 0 === $this->process->execute('git remote', $output, $this->repoDir)) {
-                $this->process->execute('git remote update --prune origin', $output, $this->repoDir);
+                if (0 !== $this->process->execute('git remote update --prune origin', $output, $this->repoDir)) {
+                    throw new \RuntimeException('Failed to update '.$this->url.', could not read packages from it' . "\n\n" .$this->process->getErrorOutput());
+                }
             } else {
                 // clean up directory and do a fresh clone into it
                 $fs = new Filesystem();