Unify single and multiple exception cases

Co-authored-by: Chris Lalancette <clalancette@gmail.com>
Signed-off-by: Timon Engelke <timon.engelke@online.de>

launch/launch/invalid_launch_file_error.py

@@ -27,16 +27,12 @@ def __init__(self, extension='', *, likely_errors=None):
                 'The launch file may have a syntax error, or its format is unknown'
             )
         else:
-            if len(self._likely_errors) == 1:
-                self._error_message = (
-                    'Caught exception when trying to load file of format [{}]: {}'
-                ).format(self._extension, self._likely_errors[0])
-            else:
-                self._error_message = (
-                    'Caught multiple exceptions when trying to load file of format [{}]:'
-                ).format(self._extension)
-                for error in self._likely_errors:
-                    self._error_message += '\n - {}'.format(error)
+            self._error_message = (
+                'Caught {} when trying to load file of format [{}]:'
+            ).format('multiple exceptions' if len(self._likely_errors) > 1 else 'exception',
+                      self._extension)
+            for error in self._likely_errors:
+                self._error_message += '\n - {}'.format(error)
 
             self.__cause__ = self._likely_errors[0]