Swap to calling Errorf instead of Fatalf from Cleanup

[JosiahWitt]

Keeps panics that are raised after the chain is built from being silenced.

For example, this used to fail the test with only Found ensure(<actual>) without chained assertion., when the error was actually a panic. Now the full panic will be shown.

someSmallSlice := []string{"one item"}
ensure("hi").Equals(someSmallSlice[10000])

[codecov-commenter]

Codecov Report

Merging #46 (550d4d6) into master (fd3a99f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #46   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files           6        6           
  Lines         437      437           
=======================================
  Hits          434      434           
  Misses          2        2           
  Partials        1        1           

Impacted Files	Coverage Δ
ensurepkg/ensurepkg.go	91.89% <100.00%> (ø)

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