Display a more helpful message when a 'signature expired' error happens.

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

Some users reported spurious "signature expired" errors. After some investigation, we found that all of them had the wrong system date set on their machine, with a date set in the past. Even if the error says that the signature is "expired", it's a signature that is not yet valid (it will be in the future).

This PR change the error message, and prompt the user to check if the system clock is set correctly, hopefully leading to a self resolution of the problem.

What is the current behavior?

The error message is signature expired.

What is the new behavior?

The error message is signature expired: is your system clock set correctly?

Does this PR introduce a breaking change, and is titled accordingly?

Other information

Supersedes #2744

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.55%. Comparing base (49c154a) to head (b4c7c5a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2750      +/-   ##
==========================================
- Coverage   67.65%   67.55%   -0.10%     
==========================================
  Files         237      237              
  Lines       22356    22358       +2     
==========================================
- Hits        15126    15105      -21     
- Misses       6036     6063      +27     
+ Partials     1194     1190       -4     

Flag	Coverage Δ
unit	67.55% <100.00%> (-0.10%)	⬇️

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