Fail psql if the tunnel fails

[msakrejda]

This is especially important since the tunnel promise can resolve even
if the authentication information is incorrect, which leads to an ugly
stack trace propagated to the client.

[msakrejda]

/cc @neovintage

[codecov-io]

Codecov Report

Merging #99 into master will decrease coverage by 0.18%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   95.97%   95.79%   -0.19%     
==========================================
  Files          42       42              
  Lines         994      998       +4     
==========================================
+ Hits          954      956       +2     
- Misses         40       42       +2

Impacted Files	Coverage Δ
lib/bastion.js	90.9% <60%> (-4.1%)	⬇️

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[camillebaldock]

thanks for this