perf: throw early if file doesn't exists

[alexander-akait]

Why:

1. fs.exists is deprecated.
2. fs.readFile also throw error if the file is not readable or not exist. It is reduce fs call.

[codecov]

Codecov Report

Merging #5 into master will decrease coverage by 15.47%.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master   #5       +/-   ##
=======================================
- Coverage   15.47%   0%   -15.48%     
=======================================
  Files           2    2               
  Lines          84   80        -4     
  Branches       13   12        -1     
=======================================
- Hits           13    0       -13     
+ Misses         71   68        -3     
- Partials        0   12       +12

Impacted Files	Coverage Δ
src/index.js	0% <0%> (-15.67%)	⬇️

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[Jessidhia]

Avoids a stat(2) call, SGTM.

Almost all of the diff is just a reindent.