[socket.streambuf.cons] Add constructor postconditions for error()

Fixes NB GB-18 (PDTS)

Author: jwakely

src/socketstreams.tex

@@ -91,7 +91,7 @@
 \effects Initializes \tcode{socket_} with \tcode{ctx}, where \tcode{ctx} is an unspecified object of class \tcode{io_context}.
 
 \pnum
-\postconditions \tcode{expiry() == time_point::max()}.
+\postconditions \tcode{expiry() == time_point::max()} and \tcode{!error()}.
 \end{itemdescr}
 
 \begin{itemdecl}
@@ -103,7 +103,7 @@
 \effects Initializes \tcode{socket_} with \tcode{std::move(s)}.
 
 \pnum
-\postconditions \tcode{expiry() == time_point::max()}.
+\postconditions \tcode{expiry() == time_point::max()} and \tcode{!error()}.
 \end{itemdescr}
 
 \begin{itemdecl}
@@ -122,6 +122,10 @@
 \item
 \tcode{rhs_a.is_open() == false}
 \item
+\tcode{error() == rhs_p.error()}
+\item
+\tcode{!rhs_a.error()}
+\item
 \tcode{expiry() == rhs_p.expiry()}
 \item
 \tcode{rhs_a.expiry() == time_point::max()}