Add required dependencies error to available_* methods

[ethanwharris]

What does this PR do?

Fixes #1125

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #1148 (a71b9c2) into master (87e683f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1148   +/-   ##
=======================================
  Coverage   89.18%   89.19%           
=======================================
  Files         285      285           
  Lines       13008    13010    +2     
=======================================
+ Hits        11601    11604    +3     
+ Misses       1407     1406    -1     

Flag	Coverage Δ
unittests	89.19% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/model.py	87.85% <100.00%> (+0.04%)	⬆️
flash/text/question_answering/model.py	93.28% <0.00%> (+0.67%)	⬆️

---

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 87e683f...a71b9c2. Read the comment docs.