Throw GAIException() for getaddrinfo errors

Created a new subclass of Exception called GAIException() that will
handle error messages from getaddrinfo() instead of letting Exception()
handle it. GAIException() will make use of gai_strerror() to map the
error code to text. On Windows, gai_strerrorW() must be used if the text
is encoded with UTF-8.

common/rdr/Exception.h

@@ -40,12 +40,17 @@ namespace rdr {
   struct SystemException : public Exception {
     int err;
     SystemException(const char* s, int err_);
-  }; 
+  };
+
+  struct GAIException : public Exception {
+    int err;
+    GAIException(const char* s, int err_);
+  };
 
   struct TimedOut : public Exception {
     TimedOut() : Exception("Timed out") {}
   };
- 
+
   struct EndOfStream : public Exception {
     EndOfStream() : Exception("End of stream") {}
   };