Hi! I noticed two minor bugs with how failure to open serialtube() is handled:

1. The exception thrown is serial.serialutil.SerialException rather than PwnlibException (which is the one thrown by all other tubes when opening fails)
2. After the program exits, another error is thrown in automatically called close() which is caused by earlier unsuccessful initialization

Here is a small test case (make sure the port is actually busy before running this):

pi@raspberrypi:~ $ cat a.py 
#!/usr/bin/python
from pwn import *
import serial
import traceback

log.info("Trying to open a serial port")
try:
    s = serialtube("/dev/serial0", 115200)
except serial.serialutil.SerialException as e:  # <- note wrong exception type here
    log.warn("Failed to open a serial port!")
    log.warn(traceback.format_exc(e))
log.info("After opening serial port")

pi@raspberrypi:~ $ PWNLIB_DEBUG=1 python a.py 
[!] Pwntools does not support 32-bit Python.  Use a 64-bit release.
[*] Trying to open a serial port
[!] Failed to open a serial port!
[!] Traceback (most recent call last):
      File "a.py", line 8, in <module>
        s = serialtube("/dev/serial0", 115200)
      File "/home/pi/.local/lib/python2.7/site-packages/pwnlib/tubes/serialtube.py", line 41, in __init__
        interCharTimeout = 0
      File "/home/pi/.local/lib/python2.7/site-packages/serial/serialutil.py", line 240, in __init__
        self.open()
      File "/home/pi/.local/lib/python2.7/site-packages/serial/serialposix.py", line 268, in open
        raise SerialException(msg.errno, "could not open port {}: {}".format(self._port, msg))
    SerialException: [Errno 16] could not open port /dev/serial0: [Errno 16] Device or resource busy: '/dev/serial0'
[*] After opening serial port
Traceback (most recent call last):
  File "/home/pi/.local/lib/python2.7/site-packages/pwnlib/tubes/serialtube.py", line 87, in close
    if self.conn:
AttributeError: 'serialtube' object has no attribute 'conn'

Additionally, documentation for the serialtube module is quite lacking (as in, there is basically no documentation and you have to look at the source code to see e.g. how convert_newlines is handled)