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

```diff @@ 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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