res_smdr_whozz: Add more detailed error logging.

Since initialization of the WHOZZ Calling? units can be finicky,
add more detailed error messages so the user has better visibility
into the actual issue, rather than just seeing a message that
the serial thread terminated.

Interestingly, as of now, it seems to be the case that the device
can be in a state where we cannot synchronize with it, but
establishing a serial connection using screen to the device works,
and then future attempts within res_smdr_whozz succeed normally.

While this hasn't been reconciled yet, the additional logging
should at least steer users in the right direction.

res/res_smdr_whozz.c

@@ -178,6 +178,7 @@ static int unloading = 0;
 #define SERIAL_READ_STRING(buf, siz, pollfirst) { \
 	bufres = serial_getline(pfd, pollfirst, buf, siz); \
 	if (bufres <= 0) { \
+		ast_log(LOG_WARNING, "Failed to read string from serial port\n"); \
 		return -1; \
 	} \
 	/* These can end in CR, but since we have a LF after, it's okay */ \
@@ -216,6 +217,7 @@ static const char *state_str_func(enum line_state state)
 
 static int serial_getline(struct pollfd *pfd, int pollfirst, char *restrict buf, size_t len)
 {
+	const char *bufstart = buf;
 	size_t total = 0;
 	for (;;) {
 		ssize_t bufres = 1;
@@ -248,6 +250,7 @@ static int serial_getline(struct pollfd *pfd, int pollfirst, char *restrict buf,
 		buf += bufres;
 		len -= bufres;
 		if (len <= 0) {
+			ast_log(LOG_ERROR, "Buffer exhausted before complete line could be read? (read so far: '%s')\n", bufstart);
 			return -1;
 		}
 	}
@@ -883,11 +886,13 @@ static int serial_monitor(void *varg)
 			}
 		} while (!unloading && serial_sync(&pfd));
 		if (unloading) {
+			ast_debug(3, "Unload requested before initialization could complete\n");
 			return -1;
 		}
 	}
 
 	if (set_settings(&pfd)) {
+		ast_log(LOG_ERROR, "Failed to initialize device with SMDR settings\n");
 		return -1;
 	}