tshark errors silently ignored

Hi,

it looks like tshark errors are silently ignored, which leads to other problems down the line. Typically, I got a "Found no signals, are you sure supports monitor mode?" error, first because:

• I didn't have the right permission to run tshark, which I fixed
• then, having run some tests in root, the dumpfile used could not be overwritten by non-root user.

In these cases, instead of getting a clear error message from the subprocess, I just got the fairly unhelpful error from the script.

I would suggest the following:

• show any error from tshark as a warning. Some errors are not a problem (I do get another one, that doesn't seem to hinder the proper flow), so we may not want to abort the process on error.
• use proper temporary files / directory, to avoid the type of collision I encountered
• maybe to use logging, to have a simpler way to manage warning / info / debug (=verbose) stuff.

I'm happy to give it a shot and submit a PR, if it's something that is deemed valuable.

Thanks!