fix exit codes

[ltalirz]

Fixes #1654

@giovannipizzi Since we are using this enum elsewhere as well, we stuck with it (meaning we have to use .value).
I could move to an IntEnum if you like.

[codecov-io]

Codecov Report

Merging #1654 into verdi will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##            verdi    #1654   +/-   ##
=======================================
  Coverage   59.14%   59.14%           
=======================================
  Files         294      294           
  Lines       34526    34526           
=======================================
  Hits        20422    20422           
  Misses      14104    14104

Impacted Files	Coverage Δ
aiida/cmdline/utils/echo.py	80.64% <50%> (ø)	⬆️

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