Improve coercion error messages

* Remove redundant falsy check for runtimeType

* Replace silent conversion to null with an invariant and a useful error message

* Add an error message for unexpected return value from resolveType

* Make sure null and undefined mean could not determine type while false or 0 indicates wrong return type

src/execution/execute.js

@@ -817,12 +817,19 @@ function completeAbstractValue(
     returnType.resolveType(result, exeContext.contextValue, info) :
     defaultResolveTypeFn(result, exeContext.contextValue, info, returnType);
 
-  if (!runtimeType) {
-    return null;
-  }
+  invariant(
+    !isNullish(runtimeType),
+    `Could not determine runtime type of value "${result}" for field ${
+      info.parentType}.${info.fieldName}.`
+  );
+  invariant(
+    runtimeType instanceof GraphQLObjectType,
+    `resolveType must return an instance of GraphQLObjectType for field ${
+      info.parentType}.${info.fieldName}, received "${runtimeType}".`
+  );
 
   const schema = exeContext.schema;
-  if (runtimeType && !schema.isPossibleType(returnType, runtimeType)) {
+  if (!schema.isPossibleType(returnType, runtimeType)) {
     throw new GraphQLError(
       `Runtime Object type "${runtimeType}" is not a possible type ` +
       `for "${returnType}".`,