Add error info if tabs are found in the yaml (#18343)

If a yaml file fails to load because of tabs being used
for formatting, detect that and show a error message
with more details.

lib/ansible/errors/__init__.py

@@ -24,7 +24,8 @@ from ansible.errors.yaml_strings import ( YAML_POSITION_DETAILS,
         YAML_COMMON_DICT_ERROR,
         YAML_COMMON_UNQUOTED_COLON_ERROR,
         YAML_COMMON_PARTIALLY_QUOTED_LINE_ERROR,
-        YAML_COMMON_UNBALANCED_QUOTES_ERROR )
+        YAML_COMMON_UNBALANCED_QUOTES_ERROR,
+        YAML_COMMON_LEADING_TAB_ERROR)
 from ansible.module_utils._text import to_native, to_text
 
 
@@ -111,6 +112,9 @@ class AnsibleError(Exception):
                     #header_line   = ("=" * 73)
                     error_message += "\nThe offending line appears to be:\n\n%s\n%s\n%s\n" % (prev_line.rstrip(), target_line.rstrip(), arrow_line)
 
+                    # TODO: There may be cases where there is a valid tab in a line that has other errors.
+                    if '\t' in target_line:
+                        error_message += YAML_COMMON_LEADING_TAB_ERROR
                     # common error/remediation checking here:
                     # check for unquoted vars starting lines
                     if ('{{' in target_line and '}}' in target_line) and ('"{{' not in target_line or "'{{" not in target_line):

lib/ansible/errors/yaml_strings.py

@@ -116,3 +116,20 @@ Could be written as:
     foo: '"bad" "wolf"'
 """
 
+YAML_COMMON_LEADING_TAB_ERROR = """\
+There appears to be a tab character at the start of the line.
+
+YAML does not use tabs for formatting. Tabs should be replaced with spaces.
+
+For example:
+    - name: update tooling
+      vars:
+        version: 1.2.3
+#    ^--- there is a tab there.
+
+Should be written as:
+    - name: update tooling
+      vars:
+        version: 1.2.3
+# ^--- all spaces here.
+"""