restore rst_invalid function

used in planemo galaxyproject/planemo#1275

has been removed here galaxyproject#14588

lib/galaxy/tool_util/linters/help.py

@@ -30,10 +30,22 @@ def lint_help(tool_xml, lint_ctx):
     if "TODO" in help_text:
         lint_ctx.warn("Help contains TODO text.", node=helps[0])
 
+    invalid_rst = rst_invalid(help_text)
+    if invalid_rst:
+        lint_ctx.warn(f"Invalid reStructuredText found in help - [{invalid_rst}].", node=helps[0])
+    else:
+        lint_ctx.valid("Help contains valid reStructuredText.", node=helps[0])
+
+
+def rst_invalid(text):
+    """
+    Predicate to determine if text is invalid reStructuredText.
+    Return False if the supplied text is valid reStructuredText or
+    a string indicating the problem.
+    """
+    invalid_rst = False
     try:
-        rst_to_html(help_text, error=True)
+        rst_to_html(text, error=True)
     except Exception as e:
-        lint_ctx.warn(f"Invalid reStructuredText found in help - [{unicodify(e)}].", node=helps[0])
-        return
-
-    lint_ctx.valid("Help contains valid reStructuredText.", node=helps[0])
+        invalid_rst = unicodify(e)
+    return invalid_rst