dist/tools/usb-serial/ttys.py: return error on empty list

If no TTY serial (matching the given filters, if any) was found, use
the exit code `1`. The idea is that simple shell scripts falling back
to alternative variants of a board can be used via

```.sh
ttys.py --most-recent --model Fooboard --vendor Footronic || \
    ttys.py --most-recent --model Barboard --vendor Bartronic
```

Just adding a regex that would accept both vendors and models would
have different semantics: If both a Fooboard and a Barboard are
attached, it would pick the most recently connected of both. The shell
expression above would always prefer a Fooboard over a Borboard.

The use case cheap Arduino clones that replace the ATmega16U2 used
as USB UART bridge with cheap single purpose chips. The original
ATmega16U2 has the advantage that it provides identification data
unique the specific Arduino board, while the clones cannot be told
apart from standalone USB UART bridges or Arduino clones of other
models. Hence, we want to pick the genuine Arduino board if connected,
and only fall back to matching cheap USB UART bridges if no genuine
Arduino board is connected.

dist/tools/usb-serial/ttys.py

@@ -176,7 +176,7 @@ def generate_filters(args):
     Generate filters for use in the filters_match function from the command
     line arguments
     """
-    result = list()
+    result = []
     if args.serial is not None:
         result.append(("serial", re.compile(r"^" + re.escape(args.serial)
                        + r"$")))
@@ -225,6 +225,9 @@ def print_ttys(args):
         else:
             ttys = []
 
+    if len(ttys) == 0:
+        sys.exit(1)
+
     print_results(args, ttys)