add better error message

Author: mr-c

cwltool/job.py

@@ -623,7 +623,14 @@ def run(self,
                 img_id = str(docker_req["dockerPull"])
                 cmd = [user_space_docker_cmd, "pull", img_id]
                 _logger.info(Text(cmd))
-                subprocess.check_call(cmd, stdout=sys.stderr)
+                try:
+                    subprocess.check_call(cmd, stdout=sys.stderr)
+                except OSError:
+                    raise WorkflowException(SourceLine(docker_req).makeError(
+                        "Either Docker container {} is not available with "
+                        "user space docker implementation {} or {} is missing "
+                        "or broken.".format(img_id, user_space_docker_cmd,
+                                            user_space_docker_cmd)))
             else:
                 raise WorkflowException(SourceLine(docker_req).makeError(
                     "Docker image must be specified as 'dockerImageId' or "

tests/test_examples.py

@@ -954,7 +954,7 @@ def test_bad_userspace_runtime(factor):
         "--user-space-docker-cmd=quaquioN", "--default-container=debian",
         get_data(test_file), get_data(job_file)])
     error_code, stdout, stderr = get_main_output(commands)
-    assert "'quaquioN' not found:" in stderr or "No such file or directory: 'quaquioN'" in stderr, stderr
+    assert "or quaquioN is missing or broken" in stderr, stderr
     assert error_code == 1
 
 @windows_needs_docker