Several tahoe status fixes

[exarkun]

Fixes https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2926

[meejah]

👍 LGTM, much cleaner! :) Please merge when builders are done

[codecov-io]

Codecov Report

Merging #502 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
+ Coverage   90.13%   90.19%   +0.06%     
==========================================
  Files         144      144              
  Lines       27449    27465      +16     
  Branches     3941     3939       -2     
==========================================
+ Hits        24740    24771      +31     
+ Misses       1965     1955      -10     
+ Partials      744      739       -5

Impacted Files	Coverage Δ
src/allmydata/scripts/tahoe_status.py	98.46% <100%> (+1.97%)	⬆️
src/allmydata/web/status.py	83.2% <100%> (+1.12%)	⬆️
src/allmydata/immutable/upload.py	95.53% <0%> (+0.27%)	⬆️

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[exarkun]

I think the slackware build failure is spurious. The failure has to do with some versioning issue:

twisted.trial.unittest.FailTest: We don't know our version, because this distribution didn't come with a _version.py and 'setup.py update_version' hasn't been run.

It looks like a test bug or an environment issue. It has nothing to do with tahoe status.