fix: use NestJS HTTP exceptions in vouching service

[Doiae9]

Fixes #68

• Replaced all new Error(...) instances with appropriate NestJS HTTP exceptions (InternalServerErrorException, BadRequestException, NotFoundException) in vouching.service.ts.
• Verified and applied the same fix in vendors.service.ts at line 63.

[EmeditWeb]

Review verdict: ✅ APPROVE with minor corrections

Correct intent and matches the code standard (surface NestJS HTTP exceptions instead of bare throw new Error(...), which would otherwise all collapse to generic 500s). The InternalServerErrorException swaps in vendors/vouching are right.

Minor corrections before merge:

1. Indentation is broken in the new vouching.service.ts block (the if (error) { ... } if (!data) { ... } section is mis-indented vs the surrounding 4-space code). npm run lint (eslint --fix) should catch it — please run it.
2. Non-English comments: // Es un fallo del sistema/BD and // Es un fallo por datos inválidos del cliente — please translate to English (or remove) to match the rest of the codebase.
3. Mis-classified exception: the !data case throws BadRequestException('Invalid vouch request data provided.'). A successful insert that returns no row is a server-side condition, not client bad input — this should be InternalServerErrorException (500), otherwise clients get a misleading 400.
4. Trailing whitespace on the new import line.
5. Resolve conflicts at src/modules/vouching/vouching.service.ts

Small stuff — approve once cleaned up.