Refactor/364 error handling patterns

[Oluwasuyi-Timilehin]

#closes #364

Summary
This PR standardizes error handling behavior in the TeachLink contract for assignment #364 by removing silent failures and enforcing explicit Result propagation in the modified bridge/rewards/access paths.

What changed
Updated contracts/teachlink/src/lib.rs contract entrypoints to return and propagate errors instead of discarding them.
add_validator, remove_validator, add_supported_chain, remove_supported_chain, set_fee_recipient now return Result<(), BridgeError>.
record_course_completion now returns Result<(), BridgeError>.
Reputation wrappers now return Result<(), ReputationError>.
Rewards wrappers now return Result (update_rewards_admin, get_rewards_admin).
Updated contracts/teachlink/src/bridge.rs to eliminate silent/ignored outcomes.
Replaced ignored get_current_nonce result with explicit propagation.
Ensured all access-control role checks are propagated and mapped to BridgeError::Unauthorized.
Replaced unwrap-based config reads in touched paths with explicit error mapping.
Updated contracts/teachlink/src/rewards.rs.
Removed .ok()-based silent ignore during test bootstrap init.
Replaced unwrap-style storage reads (TOKEN, REWARDS_ADMIN) with explicit StorageError propagation.
Made admin getter/update APIs return Result consistently.
Acceptance criteria mapping
Use Result types consistently: state-changing wrappers now expose Result instead of dropping failures.
Create specific error variants: existing module-specific errors (BridgeError, RewardsError, ReputationError) are used in propagation paths.
Ensure proper propagation: failing calls now use ?/map_err(...) in touched modules.
Eliminate silent failures: removed let _ = ... and .ok() patterns in assignment scope.
Risk / Compatibility notes
Function signatures changed for several public contract entrypoints; downstream callers/tests/SDK bindings may need to handle returned Result values where they previously ignored return values.
Behavioral change is intentional: operations now fail explicitly instead of silently succeeding.
Test plan

cargo fmt --all

cargo clippy --all-targets --all-features -- -D warnings

cargo test -p teachlink-contract

Verify no silent bridge wrapper calls remain:
rg "let _ = bridge::Bridge::" contracts/teachlink/src/lib.rs

Verify no .ok() silent ignores in rewards touched path:
rg ".ok()" contracts/teachlink/src/rewards.rs

Spot-check role check propagation in bridge module:
rg "check_role(" contracts/teachlink/src/bridge.rs

#closes

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[Xhristin3]

@Oluwasuyi-Timilehin resolve conflicts.