Port fixes from alpenhorn1

[ketiltrout]

From https://github.com/chime-experiment/alpenhorn/pull/15:

• fix BBCP command line to avoid stalls
• handle "file copy already on target" before handling "file copy missing from source"
• When copies fail, don't force a check of source files which aren't the problem

From https://github.com/chime-experiment/alpenhorn/pull/17:

• Don't force source check if rsync write fails (which can't be due to the source file and can be expensive for things like HFB files)

From https://github.com/chime-experiment/alpenhorn/pull/19:

• decode all the strings returned by run_command to avoid having to deal with bytes
• remove the unnecessary string concats followed by splitting when passing commands to run_command
• add -V to bbcp because bbcp is weirdly broken
• present stderr in "copy failed" error messages

Some of the fixes from these PRs have already been ported. Not included are CHIME-specific fixes which we'll have to figure out what to do about later.

There's going to be work to merge these changes with the task queue branch. I can take care of that.

[anjakefala]

I realise that two of my questions were because I was looking at a single commit (and had forgotten that I was). I am going to resolve those.