Filesystem copy should return error message, not error category

[kryton]

diff --git a/src/filesystem.cpp b/src/filesystem.cpp
index 1b8df20..e643097 100644
--- a/src/filesystem.cpp
+++ b/src/filesystem.cpp
@@ -257,7 +257,7 @@ namespace fc {
      }
      if( ec ) {
         FC_THROW( "Copy from ${srcfile} to ${dstfile} failed because ${reason}",
-              ("srcfile",f)("dstfile",t)("reason", ec.category().name() ) );
+              ("srcfile",f)("dstfile",t)("reason", ec.message() ) );
      }
   }
   void resize_file( const path& f, size_t t )

(thx Steve!)