Always open serial monitor triggers exception on re-enumeration

[facchinm]

The new "always-open serial monitor" feature (which helps a lot the workflow, by the way) can throw an exception if the previously open port doesn't exist anymore after flashing.
This bug is probably Linux only.

To reproduce try the following:

• connect a board with 1200bps autoreset feature (Leonardo, Due Native Port etc)
• open the serial monitor
• reset the board with Reset button ->this will trigger a re-enumeration, opened port is locked so another port will be assigned
• close the serial monitor and reopen it with the new port
• program a sketch
• at this point the serial monitor will try to re-activate, but the port is disappeared so it will throw the following exception

processing.app.SerialException: Failed opening the port
    at processing.app.AbstractMonitor.resume(AbstractMonitor.java:227)
    at processing.app.Editor$DefaultExportHandler.run(Editor.java:2558)
    at java.lang.Thread.run(Thread.java:745)
Failed opening the port

The information about the new port name is known (it is the one that reappears after 1200bps reset) so these boards will probably need to be handled differently