Fix CI failures: constexpr, NRVO, cppcheck, IWYU (#5891)

- json_sax_dom_error_state::is_errored() cannot be constexpr: the class
  is not a literal type (GCC 4.8, GCC -Wpedantic, Clang 3.5).
- to_binary_vector: destroy the vector writer before returning, as the
  output_vector_sink sets the size of the vector in its destructor.
  Without NRVO (MSVC Debug in C++14 mode), the vector was moved out
  before it was resized.
- parser::sax_parse: return false instead of result after the early exit
  (cppcheck identicalConditionAfterEarlyExit).
- json.hpp: export detail/exceptions.hpp for IWYU, so json_view.hpp can
  name exception_id without including a detail header.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -8255,7 +8255,7 @@ NLOHMANN_JSON_NAMESPACE_END
 // #include <nlohmann/detail/conversions/to_json.hpp>
 // IWYU pragma: keep
 // #include <nlohmann/detail/exceptions.hpp>
-
+// IWYU pragma: export
 // #include <nlohmann/detail/hash.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++
@@ -15390,7 +15390,7 @@ class json_sax_dom_error_state
         return false;
     }
 
-    constexpr bool is_errored() const
+    bool is_errored() const
     {
         return errored;
     }
@@ -21024,7 +21024,7 @@ class parser
             return finish_input(*sax, strict);
         }
 
-        return result;
+        return false;
     }
 
   private:
@@ -37855,8 +37855,12 @@ public:
     {
         std::vector<std::uint8_t> result;
         result.reserve(detail::binary_initial_capacity(j));
-        auto writer = vector_writer(result, error_handler);
-        write(writer);
+        {
+            // the sink sets the size of result when it is destroyed, which
+            // must happen before result is returned (without NRVO, it is moved)
+            auto writer = vector_writer(result, error_handler);
+            write(writer);
+        }
         return result;
     }