fix: revert with custom error for 0 conversion amounts

[nmlinaric]

Description

If conversion amount from source chain to Bridge internal decimals (18) would result in 0 converted amount, revert with custom error DepositAmountTooSmall

How Has This Been Tested? Testing details.

unit tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• I have commented my code, particularly in hard-to-understand areas.
• I have ensured that all acceptance criteria (or expected behavior) from issue are met
• I have updated the documentation locally and in Sygma-docs.
• I have added tests to cover my changes.
• I have ensured that all the checks are passing and green, I've signed the CLA bot