Tidy up error types:

* Restructure header material
* Clean up namespace type injections
* Remove extraneous 'success' constants

include/beast/core/async_completion.hpp

@@ -34,11 +34,11 @@ namespace beast {
     ...
     template<class CompletionHandler>
     typename async_completion<CompletionHandler,
-        void(boost::system::error_code)>::result_type
+        void(error_code)>::result_type
     async_initfn(..., CompletionHandler&& handler)
     {
         async_completion<CompletionHandler,
-            void(boost::system::error_code)> completion(handler);
+            void(error_code)> completion(handler);
         ...
         return completion.result.get();
     }

include/beast/core/detail/stream_concepts.hpp

@@ -9,6 +9,7 @@
 #define BEAST_DETAIL_STREAM_CONCEPTS_HPP
 
 #include <beast/core/buffer_concepts.hpp>
+#include <beast/core/error.hpp>
 #include <boost/asio/io_service.hpp>
 #include <boost/system/error_code.hpp>
 #include <type_traits>
@@ -22,7 +23,7 @@ namespace detail {
 struct StreamHandler
 {
     StreamHandler(StreamHandler const&) = default;
-    void operator()(boost::system::error_code ec, std::size_t);
+    void operator()(error_code ec, std::size_t);
 };
 using ReadHandler = StreamHandler;
 using WriteHandler = StreamHandler;
@@ -79,9 +80,6 @@ public:
 template<class T>
 class is_SyncReadStream
 {
-    using error_code =
-        boost::system::error_code;
-
     template<class U, class R = std::is_same<decltype(
         std::declval<U>().read_some(
             std::declval<MutableBufferSequence>())),
@@ -108,9 +106,6 @@ public:
 template<class T>
 class is_SyncWriteStream
 {
-    using error_code =
-        boost::system::error_code;
-
     template<class U, class R = std::is_same<decltype(
         std::declval<U>().write_some(
             std::declval<ConstBufferSequence>())),

include/beast/core/error.hpp

@@ -19,6 +19,18 @@ using error_code = boost::system::error_code;
 /// The type of system error thrown by the library
 using system_error = boost::system::system_error;
 
+/// The type of error category used by the library
+using error_category = boost::system::error_category;
+
+/// The type of error condition used by the library
+using error_condition = boost::system::error_condition;
+
+/// The set of constants used for cross-platform error codes
+#if GENERATING_DOCS
+enum errc{};
+#else
+namespace errc = boost::system::errc;
+#endif
 } // beast
 
 #endif