Use showText to show add-on import errors (#2103)

* Use showText to show add-on import errors

Long error messages can be completely unreadable as QMessageBox doesn't
show a scrollbar and the text can't be selected and copied by default (on Windows at least).

* HTML-escape error

qt/aqt/addons.py

@@ -3,6 +3,7 @@
 
 from __future__ import annotations
 
+import html
 import io
 import json
 import os
@@ -45,6 +46,7 @@ from aqt.utils import (
     send_to_trash,
     show_info,
     showInfo,
+    showText,
     showWarning,
     tooltip,
     tr,
@@ -246,12 +248,18 @@ class AddonManager:
             except AbortAddonImport:
                 pass
             except:
-                showWarning(
+                error = html.escape(
                     tr.addons_failed_to_load(
                         name=addon.human_name(),
                         traceback=traceback.format_exc(),
                     )
                 )
+                txt = f"<h1>{tr.qt_misc_error()}</h1><div style='white-space: pre-wrap'>{error}</div>"
+                showText(
+                    txt,
+                    type="html",
+                    copyBtn=True,
+                )
 
     def onAddonsDialog(self) -> None:
         aqt.dialogs.open("AddonsDialog", self)