Rephrased error message for unreachable join task

Rephrased the error message for unreachable join task to be more descriptive and clearer to the user why the join task is unreachable.

Author: m4dcoder

CHANGELOG.rst

@@ -4,6 +4,11 @@ Changelog
 0.6
 ---
 
+Changed
+~~~~~~~
+
+* Rephrased the error message for the unreachable join task. Fixes #162 (improvement)
+
 Fixed
 ~~~~~
 

orquesta/specs/native/v1/models.py

@@ -465,8 +465,15 @@ def detect_unreachable_tasks(self, parent=None):
                 meta['reachable'] = (meta['splits'] == staging[prev_task_name]['splits'])
 
                 if not meta['reachable']:
+                    msg = (
+                        'The join task "%s" is unreachable. A join task is determined to be '
+                        'unreachable if there are nested forks from multi-referenced tasks '
+                        'that join on the said task. This is ambiguous to the workflow engine '
+                        'because it does not know at which level should the join occurs.'
+                    )
+
                     entry = {
-                        'message': 'The join task "%s" is unreachable.' % task_name,
+                        'message': msg % task_name,
                         'spec_path': meta['spec_path'],
                         'schema_path': meta['schema_path']
                     }

orquesta/tests/unit/specs/native/test_workflow_spec_validate.py

@@ -412,7 +412,12 @@ def test_unreachable_task(self):
         expected_errors = {
             'semantics': [
                 {
-                    'message': 'The join task "task5" is unreachable.',
+                    'message': (
+                        'The join task "task5" is unreachable. A join task is determined to be '
+                        'unreachable if there are nested forks from multi-referenced tasks '
+                        'that join on the said task. This is ambiguous to the workflow engine '
+                        'because it does not know at which level should the join occurs.'
+                    ),
                     'spec_path': 'tasks.task5',
                     'schema_path': 'properties.tasks.patternProperties.^\\w+$'
                 }