Clean up push failure messaging in "arc land" slightly

Summary: Ref PHI1808. Currently, push failures are messaged awkwardly. Make this exception handling more selective and the user-facing behavior more readable.

Test Plan: Ran "arc land" against a failing remote, saw a human-readable message instead of a stack trace.

Differential Revision: https://secure.phabricator.com/D21395

src/__phutil_library_map__.php

@@ -307,6 +307,7 @@ phutil_register_library_map(array(
     'ArcanistLandCommit' => 'land/ArcanistLandCommit.php',
     'ArcanistLandCommitSet' => 'land/ArcanistLandCommitSet.php',
     'ArcanistLandEngine' => 'land/engine/ArcanistLandEngine.php',
+    'ArcanistLandPushFailureException' => 'land/exception/ArcanistLandPushFailureException.php',
     'ArcanistLandSymbol' => 'land/ArcanistLandSymbol.php',
     'ArcanistLandTarget' => 'land/ArcanistLandTarget.php',
     'ArcanistLandWorkflow' => 'workflow/ArcanistLandWorkflow.php',
@@ -1352,6 +1353,7 @@ phutil_register_library_map(array(
     'ArcanistLandCommit' => 'Phobject',
     'ArcanistLandCommitSet' => 'Phobject',
     'ArcanistLandEngine' => 'ArcanistWorkflowEngine',
+    'ArcanistLandPushFailureException' => 'Exception',
     'ArcanistLandSymbol' => 'Phobject',
     'ArcanistLandTarget' => 'Phobject',
     'ArcanistLandWorkflow' => 'ArcanistArcWorkflow',

src/land/engine/ArcanistGitLandEngine.php

@@ -491,7 +491,7 @@ final class ArcanistGitLandEngine
         $flags_argv,
         $into_commit);
       if ($err) {
-        throw new ArcanistUsageException(
+        throw new ArcanistLandPushFailureException(
           pht(
             'Submit failed! Fix the error and run "arc land" again.'));
       }
@@ -509,16 +509,10 @@ final class ArcanistGitLandEngine
       $this->newOntoRefArguments($into_commit));
 
     if ($err) {
-      throw new ArcanistUsageException(
+      throw new ArcanistLandPushFailureException(
         pht(
           'Push failed! Fix the error and run "arc land" again.'));
     }
-
-    // TODO
-    // if ($this->isGitSvn) {
-    //   $err = phutil_passthru('git svn dcommit');
-    //   $cmd = 'git svn dcommit';
-
   }
 
   protected function reconcileLocalState(

src/land/engine/ArcanistLandEngine.php

@@ -1259,7 +1259,7 @@ abstract class ArcanistLandEngine
             try {
               $this->pushChange($into_commit);
               $this->setHasUnpushedChanges(false);
-            } catch (Exception $ex) {
+            } catch (ArcanistLandPushFailureException $ex) {
 
               // TODO: If the push fails, fetch and retry if the remote ref
               // has moved ahead of us.
@@ -1280,7 +1280,8 @@ abstract class ArcanistLandEngine
                 continue;
               }
 
-              throw $ex;
+              throw new PhutilArgumentUsageException(
+                $ex->getMessage());
             }
 
             if ($need_cascade) {

src/land/engine/ArcanistMercurialLandEngine.php

@@ -836,7 +836,7 @@ final class ArcanistMercurialLandEngine
       foreach ($body as $command) {
         $err = $this->newPassthru('%Ls', $command);
         if ($err) {
-          throw new ArcanistUsageException(
+          throw new ArcanistLandPushFailureException(
             pht(
               'Push failed! Fix the error and run "arc land" again.'));
         }

src/land/exception/ArcanistLandPushFailureException.php

@@ -0,0 +1,4 @@
+<?php
+
+final class ArcanistLandPushFailureException
+  extends Exception {}