Improve error messages with backend conditionals

doc/changelog.asciidoc

@@ -80,6 +80,7 @@ Changed
   (e.g. `--logfilter '!init,destroy'`) to invert the filter.
 - `:view-source` now has a `--pygments` flag which uses the "old" way of
   rendering sources even with QtWebEngine.
+- Improved error messages when a setting needs a newer Qt version.
 
 Removed
 ~~~~~~~

qutebrowser/config/config.py

@@ -276,7 +276,8 @@ class Config(QObject):
         """Set the given option to the given value."""
         if not isinstance(objects.backend, objects.NoBackend):
             if objects.backend not in opt.backends:
-                raise configexc.BackendError(opt.name, objects.backend)
+                raise configexc.BackendError(opt.name, objects.backend,
+                                             opt.raw_backends)
 
         opt.typ.to_py(value)  # for validation
 

qutebrowser/config/configexc.py

@@ -44,9 +44,15 @@ class BackendError(Error):
 
     """Raised when this setting is unavailable with the current backend."""
 
-    def __init__(self, name, backend):
-        super().__init__("The {} setting is not available with the {} "
-                         "backend!".format(name, backend.name))
+    def __init__(self, name, backend, raw_backends):
+        if raw_backends is None or not raw_backends[backend.name]:
+            msg = ("The {} setting is not available with the {} backend!"
+                   .format(name, backend.name))
+        else:
+            msg = ("The {} setting needs {} with the {} backend!"
+                   .format(name, raw_backends[backend.name], backend.name))
+
+        super().__init__(msg)
 
 
 class NoPatternError(Error):

tests/unit/config/test_configexc.py

@@ -54,12 +54,23 @@ def test_no_autoconfig_error():
     assert str(e) == expected
 
 
-def test_backend_error():
-    e = configexc.BackendError('foo', usertypes.Backend.QtWebKit)
+@pytest.mark.parametrize('raw_backends', [
+    None,
+    {'QtWebEngine': 'Qt 5.11', 'QtWebKit': False}
+])
+def test_backend_error(raw_backends):
+    e = configexc.BackendError('foo', usertypes.Backend.QtWebKit, raw_backends)
     expected = "The foo setting is not available with the QtWebKit backend!"
     assert str(e) == expected
 
 
+def test_backend_error_condition():
+    e = configexc.BackendError('foo', usertypes.Backend.QtWebEngine,
+                               {'QtWebEngine': 'Qt 5.11', 'QtWebKit': True})
+    expected = "The foo setting needs Qt 5.11 with the QtWebEngine backend!"
+    assert str(e) == expected
+
+
 def test_no_pattern_error():
     e = configexc.NoPatternError('foo')
     expected = "The foo setting does not support URL patterns!"