Improve the quality of SSH error messages

Summary: See PHI1784. Currently, users who pass an invalid SSH command to Phabricator's SSH handler get an unhelpful error message. Make it more helpful.

Test Plan: Ran `./bin/ssh-exec` with no arguments (old, helpful error), invalid arguments (before: unhelpful error; after: helpful error), and valid arguments (old, helpful behavior).

Differential Revision: https://secure.phabricator.com/D21362

Author: epriestley

scripts/ssh/ssh-exec.php

@@ -211,21 +211,29 @@
     ->setUniqueMethod('getName')
     ->execute();
 
+  $command_list = array_keys($workflows);
+  $command_list = implode(', ', $command_list);
+
+  $error_lines = array();
+  $error_lines[] = pht('Welcome to Phabricator.');
+  $error_lines[] = pht(
+    'You are logged in as %s.',
+    $user_name);
+
   if (!$original_argv) {
-    throw new Exception(
-      pht(
-        "Welcome to Phabricator.\n\n".
-        "You are logged in as %s.\n\n".
-        "You haven't specified a command to run. This means you're requesting ".
-        "an interactive shell, but Phabricator does not provide an ".
-        "interactive shell over SSH.\n\n".
-        "Usually, you should run a command like `%s` or `%s` ".
-        "rather than connecting directly with SSH.\n\n".
-        "Supported commands are: %s.",
-        $user_name,
-        'git clone',
-        'hg push',
-        implode(', ', array_keys($workflows))));
+    $error_lines[] = pht(
+      'You have not specified a command to run. This means you are requesting '.
+      'an interactive shell, but Phabricator does not provide interactive '.
+      'shells over SSH.');
+    $error_lines[] = pht(
+      '(Usually, you should run a command like "git clone" or "hg push" '.
+      'instead of connecting directly with SSH.)');
+    $error_lines[] = pht(
+      'Supported commands are: %s.',
+      $command_list);
+
+    $error_lines = implode("\n\n", $error_lines);
+    throw new PhutilArgumentUsageException($error_lines);
   }
 
   $log_argv = implode(' ', $original_argv);
@@ -247,7 +255,20 @@
   $parsed_args = new PhutilArgumentParser($parseable_argv);
 
   if (empty($workflows[$command])) {
-    throw new Exception(pht('Invalid command.'));
+    $error_lines[] = pht(
+      'You have specified the command "%s", but that command is not '.
+      'supported by Phabricator. As received by Phabricator, your entire '.
+      'argument list was:',
+      $command);
+
+    $error_lines[] = csprintf('  $ ssh ... -- %Ls', $parseable_argv);
+
+    $error_lines[] = pht(
+      'Supported commands are: %s.',
+      $command_list);
+
+    $error_lines = implode("\n\n", $error_lines);
+    throw new PhutilArgumentUsageException($error_lines);
   }
 
   $workflow = $parsed_args->parseWorkflows($workflows);