feat: show a help message when the context's deadline passes

[knqyf263]

$ trivy image -timeout 1s sequenceiq/hadoop-docker:2.7.0
2021-04-27T20:35:06.245+0900    WARN    increase --timeout value
2021-04-27T20:35:06.245+0900    FATAL   scan error: image scan failed: failed analysis: analyze error: timeout: context deadline exceeded

[codecov]

Codecov Report

Merging #955 (8220826) into main (a00d719) will decrease coverage by 6.30%.
The diff coverage is 52.13%.

@@            Coverage Diff             @@
##             main     #955      +/-   ##
==========================================
- Coverage   68.50%   62.19%   -6.31%     
==========================================
  Files          57       65       +8     
  Lines        2210     2611     +401     
==========================================
+ Hits         1514     1624     +110     
- Misses        564      853     +289     
- Partials      132      134       +2     

Impacted Files	Coverage Δ
pkg/commands/artifact/config.go	81.81% <ø> (ø)
pkg/commands/artifact/fs.go	0.00% <0.00%> (ø)
pkg/commands/artifact/image.go	0.00% <0.00%> (ø)
pkg/commands/artifact/repository.go	0.00% <0.00%> (ø)
pkg/commands/artifact/run.go	0.00% <0.00%> (ø)
pkg/commands/artifact/wire_gen.go	0.00% <0.00%> (ø)
pkg/commands/client/config.go	79.31% <ø> (ø)
pkg/commands/client/run.go	0.00% <0.00%> (ø)
pkg/commands/client/wire_gen.go	0.00% <0.00%> (ø)
pkg/commands/config/cache.go	100.00% <ø> (ø)
... and 43 more

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