Fix GitHub API based sysl modules single file cache issue

[ChloePlanet]

Changes proposed in this pull request:

• Fix GitHub API based sysl modules single file cache issue
• Show proper error info with github api error

[codecov[bot]]

Codecov Report

Merging #1043 into master will increase coverage by 1.24%. The diff coverage is 84.78%.

@@            Coverage Diff             @@
##           master    #1043      +/-   ##
==========================================
+ Coverage   83.35%   84.59%   +1.24%     
==========================================
  Files          73       73              
  Lines       10609    10493     -116     
==========================================
+ Hits         8843     8877      +34     
+ Misses       1432     1314     -118     
+ Partials      334      302      -32     

Impacted Files	Coverage Δ
pkg/database/db_test_utils.go	83.33% <0.00%> (ø)
pkg/diagrams/plantuml.go	0.00% <0.00%> (ø)
pkg/mod/fs.go	58.44% <50.00%> (-11.37%)	:arrow_down:
pkg/importer/importer.go	63.15% <63.15%> (ø)
pkg/mermaid/datamodeldiagram/datamodeldiagram.go	61.90% <68.42%> (+3.33%)	:arrow_up:
pkg/parse/parse.go	85.01% <71.87%> (+1.03%)	:arrow_up:
pkg/parse/fs.go	90.90% <80.00%> (-9.10%)	:arrow_down:
...g/mermaid/integrationdiagram/integrationdiagram.go	83.19% <82.45%> (-1.19%)	:arrow_down:
pkg/parse/listener_impl.go	89.21% <83.58%> (-0.30%)	:arrow_down:
pkg/importer/openapi.go	86.12% <85.45%> (-3.32%)	:arrow_down:
... and 29 more