fix: program: print task-help if ParseError occurs

This will f.e. print the help of a task when it is not given its mandatory args:
```bash
> # Prints help if my_task expects mandatory positional arg
> inv my_task
```

If no name is available for the context, f.e. the core/None context,
we skip the specific task print since this means it was not a 'user' task which failed.

Signed-off-by: laurensmiers <[email protected]>

Author: laurensmiers

invoke/program.py

@@ -403,6 +403,8 @@ def run(self, argv: Optional[List[str]] = None, exit: bool = True) -> None:
             # problems.
             if isinstance(e, ParseError):
                 print(e, file=sys.stderr)
+                if e.context and e.context.name:
+                    self.print_task_help(e.context.name)
             if isinstance(e, Exit) and e.message:
                 print(e.message, file=sys.stderr)
             if isinstance(e, UnexpectedExit) and e.result.hide:

tests/program.py

@@ -653,6 +653,23 @@ def prints_help_for_task_only(self):
                 for flag in ["-h", "--help"]:
                     expect("-c decorators {} punch".format(flag), out=expected)
 
+            def prints_help_if_no_mandatory_arg(self):
+                expected = """
+Usage: invoke [--core-opts] punch [--options] [other tasks here ...]
+
+Docstring:
+  none
+
+Options:
+  -h STRING, --why=STRING   Motive
+  -w STRING, --who=STRING   Who to punch
+
+""".lstrip()
+                expected_error= """
+'punch' did not receive required positional arguments: 'who', 'why'
+""".lstrip()
+                expect("-c decorators punch", out=expected, err=expected_error)
+
             def works_for_unparameterized_tasks(self):
                 expected = """
 Usage: invoke [--core-opts] biz [other tasks here ...]