connect: add error messages when Socket connect fails (#456)

* Throw runtime_error in Socket::create

* Add messages to MessagingError

* Fix cython bindings

* Write a message to stderr instead of throwing runtime_error

messaging/messaging.cc

@@ -64,8 +64,10 @@ SubSocket * SubSocket::create(Context * context, std::string endpoint, std::stri
   if (r == 0) {
     return s;
   } else {
+    std::cerr << "Error, failed to connect SubSocket to " << endpoint << ": " << strerror(errno) << std::endl;
+
     delete s;
-    return NULL;
+    return nullptr;
   }
 }
 
@@ -87,8 +89,10 @@ PubSocket * PubSocket::create(Context * context, std::string endpoint, bool chec
   if (r == 0) {
     return s;
   } else {
+    std::cerr << "Error, failed to bind PubSocket to " << endpoint << ": " << strerror(errno) << std::endl;
+
     delete s;
-    return NULL;
+    return nullptr;
   }
 }
 

messaging/messaging_pyx.pyx

@@ -6,6 +6,7 @@ from libcpp.string cimport string
 from libcpp.vector cimport vector
 from libcpp cimport bool
 from libc cimport errno
+from libc.string cimport strerror
 from cython.operator import dereference
 
 
@@ -18,7 +19,10 @@ from .messaging cimport Event as cppEvent, SocketEventHandle as cppSocketEventHa
 
 
 class MessagingError(Exception):
-  pass
+  def __init__(self, endpoint=None):
+    suffix = "with {endpoint}" if endpoint else ""
+    message = f"Messaging failure {suffix}: {strerror(errno.errno)}"
+    super().__init__(message)
 
 
 class MultiplePublishersError(MessagingError):
@@ -184,9 +188,9 @@ cdef class SubSocket:
 
     if r != 0:
       if errno.errno == errno.EADDRINUSE:
-        raise MultiplePublishersError
+        raise MultiplePublishersError(endpoint)
       else:
-        raise MessagingError
+        raise MessagingError(endpoint)
 
   def setTimeout(self, int timeout):
     self.socket.setTimeout(timeout)
@@ -225,9 +229,9 @@ cdef class PubSocket:
 
     if r != 0:
       if errno.errno == errno.EADDRINUSE:
-        raise MultiplePublishersError
+        raise MultiplePublishersError(endpoint)
       else:
-        raise MessagingError
+        raise MessagingError(endpoint)
 
   def send(self, bytes data):
     length = len(data)