[now-init] Check for dir existence prior to download

[styfle]

This is a minor enhancement to the way now init works.

This PR will check if the folder exists prior to fetching the tar.gz from the server so that the command can fail fast.

[codecov-io]

Codecov Report

Merging #2379 into canary will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           canary   #2379   +/-   ##
======================================
  Coverage    10.9%   10.9%           
======================================
  Files         250     250           
  Lines        9031    9031           
  Branches      991     991           
======================================
  Hits          985     985           
  Misses       7945    7945           
  Partials      101     101

Impacted Files	Coverage Δ
src/commands/init/init.ts	0% <0%> (ø)	⬆️

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