fix: ACNA-1147 - better error message for context token generation

[shazron]

Description

Depends on adobe/aio-lib-ims-jwt#25
Depends on adobe/aio-lib-ims-oauth#30

Related Issue

Fixes #69

Motivation and Context

How Has This Been Tested?

npm test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[shazron]

Re-run the tests once the two associated PRs have been released.

[codecov]

Codecov Report

Merging #70 (f5ce8b5) into master (95a0787) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #70   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          386       391    +5     
  Branches        48        49    +1     
=========================================
+ Hits           386       391    +5     

Impacted Files	Coverage Δ
src/token-helper.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 95a0787...f5ce8b5. Read the comment docs.

[moritzraho]

lgtm