[DOC] Enhance error message when vocab is not found

[eric-haibin-lin]

Description

Sometimes users have typos in their dataset_name argument for model.get_model. Report existing vocabularies when a vocab is not found.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (vocab-err@b719882). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #917 into master will increase coverage by 0.75%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #917      +/-   ##
==========================================
+ Coverage   89.72%   90.47%   +0.75%     
==========================================
  Files          66       66              
  Lines        6404     6405       +1     
==========================================
+ Hits         5746     5795      +49     
+ Misses        658      610      -48

Impacted Files	Coverage Δ
src/gluonnlp/data/utils.py	70.27% <0%> (-0.48%)	⬇️
src/gluonnlp/model/sequence_sampler.py	91.69% <0%> (+16.95%)	⬆️

[mli]

Job PR-917/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-917/1/index.html