Sync spawn glib/unix and Windows messages

In particular, the exec() and CreateProcess() errors are reported
directly, but failures in other any functions, which are extremely
rare, include some descriptive text, such as "Failed to set pipe
handle to inheritable (Access denied)". The example is artificial.

src/spawn.c

@@ -334,7 +334,7 @@ static gchar *spawn_create_process_with_pipes(char *command_line, const char *wo
 			if (!CreatePipe(&hpipe[pindex[i][0]], &hpipe[pindex[i][1]], NULL, 0))
 			{
 				hpipe[pindex[i][0]] = hpipe[pindex[i][1]] = NULL;
-				failed = "CreatePipe";
+				failed = "create pipe";
 				goto leave;
 			}
 
@@ -344,21 +344,21 @@ static gchar *spawn_create_process_with_pipes(char *command_line, const char *wo
 
 				if ((*fd[i] = _open_osfhandle((intptr_t) hpipe[i], mode[i])) == -1)
 				{
-					failed = "_open_osfhandle";
+					failed = "convert pipe handle to file descriptor";
 					message = g_strdup(g_strerror(errno));
 					goto leave;
 				}
 			}
 			else if (!CloseHandle(hpipe[i]))
 			{
-				failed = "CloseHandle";
+				failed = "close pipe";
 				goto leave;
 			}
 
 			if (!SetHandleInformation(hpipe[i + 3], HANDLE_FLAG_INHERIT,
 				HANDLE_FLAG_INHERIT))
 			{
-				failed = "SetHandleInformation";
+				failed = "set pipe handle to inheritable";
 				goto leave;
 			}
 		}
@@ -371,7 +371,7 @@ static gchar *spawn_create_process_with_pipes(char *command_line, const char *wo
 	if (!CreateProcess(NULL, command_line, NULL, NULL, TRUE, pipe_io ? CREATE_NO_WINDOW : 0,
 		environment, working_directory, &startup, &process))
 	{
-		failed = "CreateProcess";
+		failed = "";  /* report the message only */
 		/* further errors will not be reported */
 	}
 	else
@@ -389,14 +389,24 @@ leave:
 	if (failed)
 	{
 		if (!message)
+		{
+			size_t len;
+
 			message = g_win32_error_message(GetLastError());
+			len = strlen(message);
 
-	#ifdef SPAWN_TEST
-		failure = g_strdup_printf("%s() failed: %s", failed, message);
-		g_free(message);
-	#else
-		failure = message;
-	#endif
+			/* unlike g_strerror(), the g_win32_error messages may include a final '.' */
+			if (len > 0 && message[len - 1] == '.')
+				message[len - 1] = '\0';
+		}
+
+		if (*failed == '\0')
+			failure = message;
+		else
+		{
+			failure = g_strdup_printf("Failed to %s (%s)", failed, message);
+			g_free(message);
+		}
 	}
 
 	if (pipe_io)