Fix raw error crashing HTML reporter, add types, and share error helper

There was one call site where we were accidentally passing a raw
Exception as a message instead of a serialised summary, which was
crashing the HTML reporter.

This commit corrects this by serialising at that site, adds more types
to Message to automatically warn for this category of issue in the
future, and pulls the stack trace formatting code out into a public
function to provide more error details in more places.

Author: Hoolean

Lib/fontbakery/checkrunner.py

@@ -16,7 +16,6 @@
 import concurrent.futures
 import inspect
 import threading
-import traceback
 from typing import Union, Tuple
 
 from fontbakery.configuration import Configuration
@@ -26,7 +25,7 @@
     Identity,
 )
 from fontbakery.message import Message
-from fontbakery.utils import is_negated
+from fontbakery.utils import is_negated, format_error
 from fontbakery.status import (
     Status,
     ERROR,
@@ -154,7 +153,7 @@ def _get_check_dependencies(
             except Exception as err:
                 if not self.catch_errors:
                     raise
-                status = Subresult(ERROR, Message("error", str(err)))
+                status = Subresult(ERROR, Message("error", format_error(err)))
                 return (status, None)
 
             if negate:
@@ -194,7 +193,9 @@ def _get_check_dependencies(
         except Exception as error:
             if not self.catch_errors:
                 raise
-            status = Subresult(ERROR, Message("failed-dependencies", error))
+            status = Subresult(
+                ERROR, Message("failed-dependencies", format_error(error))
+            )
             return (status, None)
 
     def _run_check(self, identity: Identity):
@@ -225,10 +226,7 @@ def _run_check(self, identity: Identity):
         except Exception as error:
             if not self.catch_errors:
                 raise
-            message = f"Failed with {type(error).__name__}: {error}\n```\n"
-            message += "".join(traceback.format_tb(error.__traceback__))
-            message += "\n```"
-            subresults = [(ERROR, Message("failed-check", message))]
+            subresults = [(ERROR, Message("failed-check", format_error(error)))]
 
         result.extend(
             [

Lib/fontbakery/message.py

@@ -7,7 +7,9 @@
 class Message:
     """Status messages to be yielded by FontBakeryCheck"""
 
-    def __init__(self, code, message):
+    message: str
+
+    def __init__(self, code, message: str):
         """
         code: (string|number) a check internal, unique code to describe a
               specific failure condition. A short string is preferred, it

Lib/fontbakery/utils.py

@@ -16,6 +16,7 @@
 import os
 import subprocess
 import sys
+import traceback
 from typing import Optional
 from copy import deepcopy
 
@@ -860,3 +861,12 @@ def mark_glyphs(ttFont):
             if name in class_def and class_def[name] == 3:
                 marks.append(name)
     return marks
+
+
+def format_error(error: Exception) -> str:
+    """Detail an error with name and stack trace for serialisation."""
+
+    message = f"Failed with {type(error).__name__}: {error}\n```\n"
+    message += "".join(traceback.format_tb(error.__traceback__))
+    message += "\n```"
+    return message