Log stderr when e2e ssh commands fail

[nojnhuh]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

When an SSH command fails in e2e, there isn't much context that shows up in logs, e.g. from #5844 (comment):

Failed to get logs for Machine capz-06jj60-md-0-xdh7f-vv66g, Cluster default/capz-06jj60: running command "cat /var/log/calico/cni/cni.log": Process exited with status 1

This PR adds the stderr from the command to the error message:

Failed to get logs for Machine capz-e2e-r6cvnq-aso-md-0-ktznl-zvxdx, Cluster capz-e2e-r6cvnq/capz-e2e-r6cvnq-aso: running command "cat /var/log/calico/cni/cni.log": Process exited with status 1, stderr: cat: /var/log/calico/cni/cni.log: Permission denied

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

NONE

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.93%. Comparing base (c11dab9) to head (6fbd5bf).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5868      +/-   ##
==========================================
- Coverage   46.94%   46.93%   -0.01%     
==========================================
  Files         279      279              
  Lines       29687    29687              
==========================================
- Hits        13936    13934       -2     
- Misses      14938    14940       +2     
  Partials      813      813              

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[nojnhuh]

Getting rid of the intermediate stdoutBuf is a drive-by optimization to save us from holding onto the entire file content in memory before writing it to the file.

[marosset]

/approve

[nojnhuh]

/retest

[nojnhuh]

#5690

/retest

[mboersma]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3416a12d3352b66c3b61f0b44954801300442b63

[mboersma]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: marosset, mboersma

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