Class File now validates relative paths

The code from function validate in class UserStyleSheet has been migrated to
class File. One test had to be modified due to different expected behaviour.

qutebrowser/config/configtypes.py

@@ -818,6 +818,10 @@ class File(BaseType):
                 raise configexc.ValidationError(value, "may not be empty!")
         value = os.path.expanduser(value)
         try:
+            if not os.path.isabs(value):
+                abspath = os.path.join(standarddir.config(), value)
+                if os.path.isfile(abspath):
+                    return
             if not os.path.isfile(value):
                 raise configexc.ValidationError(value, "must be a valid file!")
             if not os.path.isabs(value):
@@ -1178,23 +1182,21 @@ class UserStyleSheet(File):
         value = os.path.expandvars(value)
         value = os.path.expanduser(value)
         try:
-            if not os.path.isabs(value):
-                abspath = os.path.join(standarddir.config(), value)
-                if os.path.isfile(abspath):
+            super().validate(value)
+        except configexc.ValidationError:
+            try:
+                if not os.path.isabs(value):
+                    # probably a CSS, so we don't handle it as filename.
+                    # FIXME We just try if it is encodable, maybe we should
+                    # validate CSS?
+                    # https://github.com/The-Compiler/qutebrowser/issues/115
+                    try:
+                        value.encode('utf-8')
+                    except UnicodeEncodeError as e:
+                        raise configexc.ValidationError(value, str(e))
                     return
-                # probably a CSS, so we don't handle it as filename.
-                # FIXME We just try if it is encodable, maybe we should
-                # validate CSS?
-                # https://github.com/The-Compiler/qutebrowser/issues/115
-                try:
-                    value.encode('utf-8')
-                except UnicodeEncodeError as e:
-                    raise configexc.ValidationError(value, str(e))
-                return
-            elif not os.path.isfile(value):
-                raise configexc.ValidationError(value, "must be a valid file!")
-        except UnicodeEncodeError as e:
-            raise configexc.ValidationError(value, e)
+            except UnicodeEncodeError as e:
+                raise configexc.ValidationError(value, e)
 
 
 class AutoSearch(BaseType):

tests/config/test_configtypes.py

@@ -1378,8 +1378,7 @@ class TestFile:
         os_path.expanduser.side_effect = lambda x: x
         os_path.isfile.return_value = True
         os_path.isabs.return_value = False
-        with pytest.raises(configexc.ValidationError):
-            self.t.validate('foobar')
+        self.t.validate('foobar')
 
     def test_validate_expanduser(self, os_path):
         """Test if validate expands the user correctly."""