Add has_error() to expected (#171)

Author: Bronek

include/pfn/expected.hpp

@@ -674,6 +674,7 @@ template <class T, class E> class expected {
   constexpr T &&operator*() && noexcept { return ::std::move(*(this->operator->())); }
   constexpr explicit operator bool() const noexcept { return set_; }
   constexpr bool has_value() const noexcept { return set_; }
+  constexpr bool has_error() const noexcept { return !set_; }
   constexpr T const &value() const &
   {
     static_assert(::std::is_copy_constructible_v<E>);
@@ -1248,6 +1249,7 @@ class expected<T, E> {
   // [expected.void.obs], observers
   constexpr explicit operator bool() const noexcept { return set_; }
   constexpr bool has_value() const noexcept { return set_; }
+  constexpr bool has_error() const noexcept { return !set_; }
   constexpr void operator*() const noexcept { ASSERT(set_); }
   constexpr void value() const &
   {