Allow function aliases to be registered for fixing problems, and add some more argument checking for fixing problems

test/test_ale_fix.vader

@@ -37,6 +37,7 @@ After:
   delfunction DoNothing
   delfunction CatLine
   delfunction ReplaceWithTempFile
+  call ale#fix#registry#ResetToDefaults()
 
 Given testft (A file with three lines):
   a
@@ -126,3 +127,18 @@ Expect(The first function should be used):
   ^a
   ^b
   ^c
+
+Execute(ALEFix should complain for missing functions):
+  let g:ale_fixers.testft = ['XXX', 'YYY']
+  AssertThrows ALEFix
+  AssertEqual 'Vim(echoerr):Invalid fixers used: [''XXX'', ''YYY'']', g:vader_exception
+
+Execute(ALEFix should use functions from the registry):
+  call ale#fix#registry#Add('add_carets', 'AddCarets', [], 'Add some carets')
+  let g:ale_fixers.testft = ['add_carets']
+  ALEFix
+
+Expect(The registry function should be used):
+  ^a
+  ^b
+  ^c