Display error on qt_mainloop crashes with old Qt.

See #447.

qutebrowser/app.py

@@ -223,7 +223,7 @@ class Application(QApplication):
                 if data:
                     # Crashlog exists and has data in it, so something crashed
                     # previously.
-                    self._crashdlg = crashdialog.FatalCrashDialog(
+                    self._crashdlg = crashdialog.get_fatal_crash_dialog(
                         self._args.debug, data)
                     self._crashdlg.show()
             else:

qutebrowser/misc/crashdialog.py

@@ -30,10 +30,10 @@ import traceback
 from PyQt5.QtCore import pyqtSlot, Qt, QSize, qVersion
 from PyQt5.QtWidgets import (QDialog, QLabel, QTextEdit, QPushButton,
                              QVBoxLayout, QHBoxLayout, QCheckBox,
-                             QDialogButtonBox)
+                             QDialogButtonBox, QMessageBox)
 
 import qutebrowser
-from qutebrowser.utils import version, log, utils, objreg
+from qutebrowser.utils import version, log, utils, objreg, qtutils
 from qutebrowser.misc import miscwidgets
 from qutebrowser.browser.network import pastebin
 from qutebrowser.config import config
@@ -63,6 +63,35 @@ def parse_fatal_stacktrace(text):
         return (m.group(1), m.group(2))
 
 
+def get_fatal_crash_dialog(debug, data):
+    """Get a fatal crash dialog based on a crash log.
+
+    If the crash is a segfault in qt_mainloop and we're on an old Qt version
+    this is a simple error dialog which lets the user know they should upgrade
+    if possible.
+
+    If it's anything else, it's a normal FatalCrashDialog with the possibility
+    to report the crash.
+
+    Args:
+        debug: Whether the debug flag (--debug) was given.
+        data: The crash log data.
+    """
+    errtype, frame = parse_fatal_stacktrace(data)
+    if (qtutils.version_check('5.4') or errtype != 'Segmentation fault' or
+            frame != 'qt_mainloop'):
+        return FatalCrashDialog(debug, data)
+    else:
+        title = "qutebrowser was restarted after a fatal crash!"
+        text = ("<b>qutebrowser was restarted after a fatal crash!</b><br/>"
+                "Unfortunately, this crash occured in Qt (the library "
+                "qutebrowser uses), and your version ({}) is outdated - "
+                "Qt 5.4 or later is recommended. Unfortuntately Debian and "
+                "Ubuntu don't ship a newer version (yet?)...".format(
+                    qVersion()))
+        return QMessageBox(QMessageBox.Critical, title, text, QMessageBox.Ok)
+
+
 class _CrashDialog(QDialog):
 
     """Dialog which gets shown after there was a crash.

qutebrowser/qutebrowser.py

@@ -110,7 +110,11 @@ def main():
     app = app.Application(args)
 
     def qt_mainloop():
-        """Simple wrapper to get a nicer stack trace for segfaults."""
+        """Simple wrapper to get a nicer stack trace for segfaults.
+
+        WARNING: misc/crashdialog.py checks the stacktrace for this function
+        name, so if this is changed, it should be changed there as well!
+        """
         return app.exec_()  # pylint: disable=maybe-no-member
 
     # We set qApp explicitely here to reduce the risk of segfaults while