[2264] Addressed review comments.

Removed obsoleted error messages, altered exception by Packet6::packTCP to
be NotImplemented rather than Unexpected.  Corrected method commentary.

src/bin/dhcp4/dhcp4_messages.mes

@@ -149,11 +149,6 @@ This error is output if the IPv4 DHCP server fails to send an assembled
 DHCP message to a client. The reason for the error is included in the
 message.
 
-% DHCP4_PACK_FAIL failed to assemble response correctly
-This error is output if the server failed to assemble the data to be
-returned to the client into a valid packet.  The cause is most likely
-to be a programming error: please raise a bug report.
-
 % DHCP4_PARSER_COMMIT_EXCEPTION parser failed to commit changes
 On receipt of message containing details to a change of the IPv4 DHCP
 server configuration, a set of parsers were successfully created, but one

src/bin/dhcp6/dhcp6_messages.mes

@@ -165,11 +165,6 @@ This error is output if the IPv6 DHCP server fails to send an assembled
 DHCP message to a client. The reason for the error is included in the
 message.
 
-% DHCP6_PACK_FAIL failed to assemble response correctly
-This error is output if the server failed to assemble the data to be
-returned to the client into a valid packet.  The reason is most likely
-to be to a programming error: please raise a bug report.
-
 % DHCP6_PARSER_COMMIT_EXCEPTION parser failed to commit changes
 On receipt of message containing details to a change of the IPv6 DHCP
 server configuration, a set of parsers were successfully created, but one

src/lib/dhcp/pkt6.cc

@@ -261,7 +261,7 @@ Pkt6::packUDP() {
 void
 Pkt6::packTCP() {
     /// TODO Implement this function.
-    isc_throw(Unexpected, "DHCPv6 over TCP (bulk leasequery and failover)"
+    isc_throw(NotImplemented, "DHCPv6 over TCP (bulk leasequery and failover)"
               "not implemented yet.");
 }
 

src/lib/dhcp/pkt6.h

@@ -117,7 +117,7 @@ public:
     /// will be set in data_len_.
     ///
     /// @throw BadValue if packet protocol is invalid, InvalidOperation
-    /// if packing fails, or Unexpected if protocol is TCP (IPv6 over TCP is 
+    /// if packing fails, or NotImplemented if protocol is TCP (IPv6 over TCP is
     /// not yet supported).
     void pack();
 
@@ -417,12 +417,12 @@ protected:
     ///
     /// TODO This function is not implemented yet.
     ///
-    /// Method with throw exception if build fails
+    /// @throw NotImplemented, IPv6 over TCP is not yet supported.
     void packTCP();
 
     /// Builds on wire packet for UDP transmission.
     ///
-    /// Method with throw exception if build fails
+    /// @throw InvalidOperation if packing fails
     void packUDP();
 
     /// @brief Parses on-wire form of TCP DHCPv6 packet.