More verbose errors for NoManifestExceptions.

The old "manifest required for this command -- please run
init" is replaced by a more helpful message that lists the
command repo was trying to execute (with arguments) as well
as the str() of the NoManifestException. For example:

> error: in `sync`: [Errno 2] No such file or directory:
> 	'path/to/.repo/manifests/.git/HEAD'
> error: manifest missing or unreadable -- please run init

Other failure points in basic command parsing and dispatch
are more clearly explained in the same fashion.

Change-Id: I6212e5c648bc5d57e27145d55a5391ca565e4149

error.py

@@ -24,6 +24,13 @@ class ManifestInvalidRevisionError(Exception):
 class NoManifestException(Exception):
   """The required manifest does not exist.
   """
+  def __init__(self, path, reason):
+    super(NoManifestException, self).__init__()
+    self.path = path
+    self.reason = reason
+
+  def __str__(self):
+    return self.reason
 
 class EditorError(Exception):
   """Unspecified error from the user's text editor.

main.py

@@ -129,8 +129,15 @@ class _Repo(object):
             file=sys.stderr)
       return 1
 
+    try:
       copts, cargs = cmd.OptionParser.parse_args(argv)
       copts = cmd.ReadEnvironmentOptions(copts)
+    except NoManifestException as e:
+      print('error: in `%s`: %s' % (' '.join([name] + argv), str(e)),
+        file=sys.stderr)
+      print('error: manifest missing or unreadable -- please run init',
+            file=sys.stderr)
+      return 1
 
     if not gopts.no_pager and not isinstance(cmd, InteractiveCommand):
       config = cmd.manifest.globalConfig
@@ -146,14 +153,12 @@ class _Repo(object):
     start = time.time()
     try:
       result = cmd.Execute(copts, cargs)
-    except DownloadError as e:
-      print('error: %s' % str(e), file=sys.stderr)
-      result = 1
-    except ManifestInvalidRevisionError as e:
-      print('error: %s' % str(e), file=sys.stderr)
-      result = 1
-    except NoManifestException as e:
-      print('error: manifest required for this command -- please run init',
+    except (DownloadError, ManifestInvalidRevisionError,
+        NoManifestException) as e:
+      print('error: in `%s`: %s' % (' '.join([name] + argv), str(e)),
+        file=sys.stderr)
+      if isinstance(e, NoManifestException):
+        print('error: manifest missing or unreadable -- please run init',
               file=sys.stderr)
       result = 1
     except NoSuchProjectError as e:

project.py

@@ -2327,8 +2327,8 @@ class Project(object):
         path = os.path.join(self._project.worktree, '.git', HEAD)
       try:
         fd = open(path, 'rb')
-      except IOError:
-        raise NoManifestException(path)
+      except IOError as e:
+        raise NoManifestException(path, str(e))
       try:
         line = fd.read()
       finally: