feat: improve `Assignment` API errors

[guorong009]

Description

Improve the Assignment API error messages by:

• replace simple assertion with error return
• use the cell name/description for better message

Related issues

• Resolve #253

Changes

• create AssignmentError variant & enum for FrontendError enum
• replace the assertion check with AssignmentError return in MockProver & WitnessCollection impls

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 15.10791% with 118 lines in your changes missing coverage. Please review.

Project coverage is 82.14%. Comparing base (26fc7c2) to head (04d09b1). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
halo2_frontend/src/plonk/error.rs	0.00%	64 Missing :warning:
halo2_frontend/src/dev.rs	32.78%	41 Missing :warning:
halo2_frontend/src/circuit.rs	7.14%	13 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
- Coverage   82.64%   82.14%   -0.50%     
==========================================
  Files          84       84              
  Lines       17750    17870     +120     
==========================================
+ Hits        14669    14680      +11     
- Misses       3081     3190     +109     

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