fix: raise better error when source content unexpected

[antazoey]

What I did

fixes: #4523

How I did it

Insteat of a mypy assert, raise a more helpful ValueError

How to verify it

Compile a Vyper file with ABI json for the content.
Notice error improvement.

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

improve error message when a .vy. source file content is ABI JSON

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.22%. Comparing base (579dd57) to head (1314595).

Files with missing lines	Patch %	Lines
vyper/cli/vyper_compile.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4524      +/-   ##
==========================================
- Coverage   92.24%   92.22%   -0.02%     
==========================================
  Files         123      123              
  Lines       17500    17501       +1     
  Branches     2960     2961       +1     
==========================================
- Hits        16142    16141       -1     
- Misses        946      947       +1     
- Partials      412      413       +1     

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