Improve error message for unwrapped Collections

[marcgs]

Currently the error message for a Class having a collection as a field which is initialized in constructor is: "Attempts to wrap mutable collection type using a non-whitelisted unmodifiable wrapper method". This is misleading, since in this case nothing is getting wrapped at all.

The goal of this issue is to improve this error message to something like "Not wrapped collection type". This mutability reason is detected by MutableTypeToFieldChecker.

Here a test which asserts the current error message

   public final static class ClassNotWrappingCollection {
        private final Collection<String> collection;

        public ClassNotWrappingCollection(Collection<String> collection) {
            this.collection = collection;
        }
    }

    @Test
    public void isImmutableMutableTypeToField_ClassNotWrappingCollection() throws Exception {
        try {
            assertImmutable(ClassNotWrappingCollection.class);
            fail("Error should be thrown");
        } catch (MutabilityAssertionError e) {
            assertThat(e.getMessage(), is("\n"+
                    "Expected: org.mutabilitydetector.ErrorLocationTest$ClassNotWrappingCollection to be IMMUTABLE\n" +
                    "     but: org.mutabilitydetector.ErrorLocationTest$ClassNotWrappingCollection is actually NOT_IMMUTABLE\n" +
                    "    Reasons:\n" +
                    "        Attempts to wrap mutable collection type using a non-whitelisted unmodifiable wrapper method. [Field: collection, Class: org.mutabilitydetector.ErrorLocationTest$ClassNotWrappingCollection]\n" +
                    "    Allowed reasons:\n" +
                    "        None."));
        }
    }

[Grundlefleck]

Thanks, another to apply the newcomer tag to.

[Grundlefleck]

Now not so misleading, thanks @kwahsog!