Property mapping error handling

src/lib/PropertyMapper.vala

@@ -1,6 +1,6 @@
 namespace Invercargill {
 
-    public delegate TProp PropertyGetter<TClass, TProp>(TClass object);
+    public delegate TProp PropertyGetter<TClass, TProp>(TClass object) throws Error;
     public delegate bool PropertyPredicate<TClass>(TClass object);
     public delegate void PropertySetter<TClass, TProp>(TClass object, TProp value) throws Error;
     public delegate void PropertyDefaultSetter<TClass>(TClass object);
@@ -34,11 +34,15 @@ namespace Invercargill {
                     continue;
                 }
                 if(mapping.undefined_setter == null && !exists){
-                    throw new IndexError.KEY_NOT_FOUND(@"Failed to map into $(typeof(T).name()): Mandatory property \"$(mapping.name)\" was not present in the properties list");
+                    throw new IndexError.KEY_NOT_FOUND(@"Failed to map into $(typeof(T).name()): Property \"$(mapping.name)\" does not have an undefined setter and no matching property was found");
                 }
                 var element = properties[mapping.name];
-                if(element.is_null()) {
-
+                if(mapping.null_setter != null && element.is_null()) {
+                    mapping.null_setter(object);
+                    continue;
+                }
+                if(mapping.null_setter == null && element.is_null()) {
+                    throw new ElementError.INVALID_CONVERSION(@"Failed to map into $(typeof(T).name()): Property \"$(mapping.name)\" does not have a null setter and a null value was found");
                 }
                 mapping.setter(object, properties[mapping.name]);
             }
@@ -50,7 +54,7 @@ namespace Invercargill {
             return obj;
         }
 
-        public override Properties map_from(T object) {
+        public override Properties map_from(T object) throws Error {
             var properties = new PropertiesDictionary();
             foreach (var mapping in mappings) {
                 // Check undefined
@@ -144,7 +148,7 @@ namespace Invercargill {
         public virtual PropertyMappingBuilder<T> map_many_with<TNative, TElement>(string name, owned PropertyGetter<T, Enumerable<TNative>> getter, owned PropertySetter<T, Enumerable<TNative>> setter, Mapper<TNative, TElement> mapper) {
             var mapping = new PropertyMapping<T>() {
                 name = name,
-                getter = (o) => new NativeElement<Elements>(getter(o).select<TElement>(i => mapper.map_from(i)).to_elements()),
+                getter = (o) => new NativeElement<Elements>(getter(o).try_select<TElement>(i => mapper.map_from(i)).to_elements()),
                 setter = (o,d) => {
                     var collection = new Series<TNative>();
                     foreach (var properties in d.as<Elements>().elements_as<TElement>()) {