feat: improve error handling(Error::Synthesis)

[guorong009]

Description

• Improve the error handling related to Error::Synthesis

Related issues

• Resolve Improve error message experience #219

Change

• Add WitnessMissing variant to AssignError

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 25.00000% with 36 lines in your changes missing coverage. Please review.

Project coverage is 84.99%. Comparing base (81c7058) to head (3baa282).

Files with missing lines	Patch %	Lines
halo2_frontend/src/dev.rs	17.64%	14 Missing ⚠️
halo2_frontend/src/plonk/error.rs	0.00%	8 Missing ⚠️
halo2_frontend/src/plonk/keygen.rs	42.85%	8 Missing ⚠️
halo2_frontend/src/circuit.rs	33.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #368      +/-   ##
==========================================
- Coverage   85.07%   84.99%   -0.09%     
==========================================
  Files          85       85              
  Lines       18658    18684      +26     
==========================================
+ Hits        15874    15880       +6     
- Misses       2784     2804      +20     

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[davidnevadoc]

LGTM

[adria0]

LGTM