Fix bug with hanging app on startup

[jesseduffield]

This PR kills two birds with one stone.
bird #1: the app hanging when the docker daemon was not set (actually happened whenever there was an error before the gui started polling events)
bird #2: the process kill function now checks to see if the process is actually there before trying to kill it

[codecov-io]

Codecov Report

Merging #8 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   18.12%   18.09%   -0.04%     
==========================================
  Files          12       12              
  Lines        1048     1050       +2     
==========================================
  Hits          190      190              
- Misses        852      854       +2     
  Partials        6        6

Impacted Files	Coverage Δ
pkg/commands/os.go	32.24% <0%> (-0.36%)	⬇️

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