Clean ups and simplify SynthesisError

[Pratyush]

Description

Reduces the size of SynthesisError to reduce overhead. Generally, we expect the happy case (no error), so it makes sense to optimize for that.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[Copilot]

Pull Request Overview

This PR cleans up and simplifies the SynthesisError implementation and related API methods throughout the codebase to reduce overhead and improve clarity.

• Removed several rarely used variants from the SynthesisError enum and streamlined error messages.
• Updated the get_lc function in both the constraint system reference and the constraint system to return an Option instead of a Result.
• Simplified helper functions such as get_all_predicate_arities by replacing custom hasher instantiation with the default.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
relations/src/utils/error.rs	Removed error variants and simplified error message formatting.
relations/src/gr1cs/constraint_system_ref.rs	Modified get_all_predicate_arities and updated get_lc to return Option.
relations/src/gr1cs/constraint_system.rs	Refactored get_lc to return Option and adjusted constraint trace logic.

Comments suppressed due to low confidence (2)

relations/src/gr1cs/constraint_system_ref.rs:422

• Changing the function signature from Result to Option removes explicit error context; ensure that all call sites are updated accordingly to handle the absence of a constraint system.

pub fn get_lc(&self, lc_index: LcIndex) -> Option<LinearCombination<F>> {

relations/src/gr1cs/constraint_system.rs:599

• Updating the return type from Result to Option hides error details; verify that the calling code properly handles the None case to avoid unintended silent failures.

pub fn get_lc(&self, lc_index: LcIndex) -> Option<&LinearCombination<F>> {