improve pty error

[squell]

This make it clear that the error that occurs when a PTY cannot be opened pertains to the PTY, and not to the command itself, i.e. it will produce:

sudo-rs: cannot execute '/usr/bin/ls': unable to open pty

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (328be1e) 57.46% compared to head (e544180) 57.36%.
Report is 11 commits behind head on main.

Files	Patch %	Lines
src/exec/use_pty/parent.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #807      +/-   ##
==========================================
- Coverage   57.46%   57.36%   -0.11%     
==========================================
  Files          74       74              
  Lines       10523    10559      +36     
==========================================
+ Hits         6047     6057      +10     
- Misses       4476     4502      +26     

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[squell]

Note: needs some work to figure out why the compliance test is failing.

Edit Passes the compliance tests locally. 🤔

Edit Seems just a GitHub issue. Very strange.