Make all methods that may fail return an Attempt

[Baptouuuu]

It's either Attempt<Process> when executing a command or Attempt<SideEffect> for everything else.

This is motivated to allow more errors being returned and not directly the one that can happen related to the process. For example a process may not start.

[codecov]

Codecov Report

Attention: Patch coverage is 99.03846% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.22%. Comparing base (8df354d) to head (7b60078).

Files with missing lines	Patch %	Lines
tests/Server/Processes/UnixTest.php	92.30%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #18      +/-   ##
=============================================
+ Coverage      97.08%   97.22%   +0.13%     
  Complexity       312      312              
=============================================
  Files             58       58              
  Lines           2061     2089      +28     
=============================================
+ Hits            2001     2031      +30     
+ Misses            60       58       -2     

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