Fix panic in agent

[dperny]

Previously, there was an issue where the agent could panic while attempting to determine if an error was temporary.

Before the change, in agent/exec/errors.go, the function IsTemporary attempted to drill down to a root cause by iterating through the causes of an error by calling errors.Cause. If an error has no cause, then errors.Cause returns that same error.

The issue is that somewhere in the depths of some code, it was possible for the error to have an underlying type that was non-comparable; for example, maps and slices are uncomparable types. This would cause a panic, as the uncomparable type cannot be compared even to itself.

However, one can see that errors.Cause has its own loop, and drills down to the root cause in its own way. There is no need for us to iterate here.

Instead, we can just take a look at the error itself, and then take a look at its cause once. If neither is temporary, the error is not temporary, and we have nothing to worry about.

[codecov]

Codecov Report

Merging #3024 (39a4233) into master (3cd1651) will increase coverage by 0.09%.
The diff coverage is 60.22%.

@@            Coverage Diff             @@
##           master    #3024      +/-   ##
==========================================
+ Coverage   62.27%   62.37%   +0.09%     
==========================================
  Files         142      155      +13     
  Lines       22888    24531    +1643     
==========================================
+ Hits        14254    15301    +1047     
- Misses       7130     7622     +492     
- Partials     1504     1608     +104