improve string representation of timeout errors

[marten-seemann]

Currently, an idle timeout's string representation is NO_ERROR: No recent network activity. With this PR, it will be Timeout: No recent network activity.

Note that this will break some applications building on top of quic-go that are performing string matching to detect timeout errors. I don't see this as a problem with this PR though, I regard this as a feature.
While it is true that our errors are not as helpful as they could be (see #2441), detecting timeout errors is not a problem: Our errors implement the net.Error interface, which defines a Timeout() bool method.

[codecov]

Codecov Report

Merging #3118 (afbb2b7) into master (81d16a9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3118      +/-   ##
==========================================
+ Coverage   85.47%   85.48%   +0.01%     
==========================================
  Files         131      131              
  Lines        9650     9654       +4     
==========================================
+ Hits         8248     8252       +4     
  Misses       1034     1034              
  Partials      368      368              

Impacted Files	Coverage Δ
internal/qerr/quic_error.go	97.73% <100.00%> (+0.23%)	⬆️

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