bugdown: Trigger test failure for invalid Markdown include statements.

This commit adds a custom Markdown include extension which is
identical to the original except when a macro file can't
be found, it raises a custom JsonableError exception, which
we can catch and then trigger an appropriate test failure.

Fixes: #10947

zerver/lib/bugdown/include.py

@@ -0,0 +1,67 @@
+from __future__ import print_function
+import re
+import os
+from typing import Any, Dict, Optional, List
+
+import markdown
+from markdown_include.include import MarkdownInclude, IncludePreprocessor
+
+from zerver.lib.exceptions import InvalidMarkdownIncludeStatement
+
+INC_SYNTAX = re.compile(r'\{!\s*(.+?)\s*!\}')
+
+
+class MarkdownIncludeCustom(MarkdownInclude):
+    def extendMarkdown(self, md: markdown.Markdown, md_globals: Dict[str, Any]) -> None:
+        md.preprocessors.add(
+            'include_wrapper',
+            IncludeCustomPreprocessor(md, self.getConfigs()),
+            '_begin'
+        )
+
+class IncludeCustomPreprocessor(IncludePreprocessor):
+    """
+    This is a custom implementation of the markdown_include
+    extension that checks for include statements and if the included
+    macro file does not exist or can't be opened, raises a custom
+    JsonableError exception. The rest of the functionality is identical
+    to the original markdown_include extension.
+    """
+    def run(self, lines: List[str]) -> List[str]:
+        done = False
+        while not done:
+            for line in lines:
+                loc = lines.index(line)
+                m = INC_SYNTAX.search(line)
+
+                if m:
+                    filename = m.group(1)
+                    filename = os.path.expanduser(filename)
+                    if not os.path.isabs(filename):
+                        filename = os.path.normpath(
+                            os.path.join(self.base_path, filename)
+                        )
+                    try:
+                        with open(filename, 'r', encoding=self.encoding) as r:
+                            text = r.readlines()
+                    except Exception as e:
+                        print('Warning: could not find file {}. Error: {}'.format(filename, e))
+                        lines[loc] = INC_SYNTAX.sub('', line)
+                        raise InvalidMarkdownIncludeStatement(m.group(0).strip())
+
+                    line_split = INC_SYNTAX.split(line)
+                    if len(text) == 0:
+                        text.append('')
+                    for i in range(len(text)):
+                        text[i] = text[i].rstrip('\r\n')
+                    text[0] = line_split[0] + text[0]
+                    text[-1] = text[-1] + line_split[2]
+                    lines = lines[:loc] + text + lines[loc+1:]
+                    break
+            else:
+                done = True
+
+        return lines
+
+def makeExtension(*args: Any, **kwargs: str) -> MarkdownIncludeCustom:
+    return MarkdownIncludeCustom(kwargs)