add unit test and fix possible bugs

[johnnychen94]

The 0.1.0 version is completed with no unit test at all and it's time to pay my debt...

Summary:

• timeout_call behavior is corrected to better handle exception cases in beb9f80
• with_cache_dir is enhanced to better handle exception cases in 457300a

Todo:

• [x] update usage of timeout_call and print more informative error message ~- [ ] verify if the cache is valid using by keyword~

[codecov[bot]]

Codecov Report

Merging #6 into master will increase coverage by 24.17%. The diff coverage is 96.77%.

@@             Coverage Diff             @@
##           master       #6       +/-   ##
===========================================
+ Coverage    0.32%   24.49%   +24.17%     
===========================================
  Files           6        6               
  Lines         310      298       -12     
===========================================
+ Hits            1       73       +72     
+ Misses        309      225       -84     

Impacted Files	Coverage Δ
src/mirror_tarball.jl	0.00% <0.00%> (ø)
src/utils/server_utils.jl	45.54% <100.00%> (+45.54%)	:arrow_up:
src/utils/utils.jl	100.00% <100.00%> (+100.00%)	:arrow_up:
src/utils/registry_utils.jl	0.00% <0.00%> (ø)
src/types.jl	13.33% <0.00%> (+13.33%)	:arrow_up:

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