Update flow def for AssertContext.throws to match typescript def (#1443)

Having a specific promise resolution value of Error was causing issues with methods that throw a custom error object. Fixes #1442

Author: dancoates

index.js.flow

@@ -58,8 +58,8 @@ type AssertContext = {
 // Assert that function throws an error or promise rejects.
 // @param error Can be a constructor, regex, error message or validation function.
 throws: {
-(value: PromiseLike<mixed>, error?: ErrorValidator, message?: string): Promise<Error>;
-(value: () => mixed, error?: ErrorValidator, message?: string): Error;
+(value: PromiseLike<mixed>, error?: ErrorValidator, message?: string): Promise<any>;
+(value: () => mixed, error?: ErrorValidator, message?: string): any;
 };
 // Assert that function doesn't throw an error or promise resolves.
 notThrows: {