Fee amount validation

**Bug Description**
Most non-virtual operations have a check `FC_ASSERT(fee.amount >= 0)` in `validate()` function. But some are different.
* `blind_transfer_operation` doesn't check `fee.amount`;
* `proposal_create_operation` doesn't check `fee.amount`;
* `custom_operation` checks `fee.amount > 0`, which is practically fine but not perfect;
* `balance_claim_operation` checks `fee.amount == asset()`, which is fine.

Anyway, there is a check `FC_ASSERT( fee.amount >= 0 )` in `generic_evaluator::prepare_fee()`, so the chain won't allow any user to pay a negative fee, which means we're safe. However, since the check is done in evaluators, there exists a minor issue that a user can wrap an operation with a negative fee in a `proposal_create_operation` which could be accepted by the chain, just a bit annoying.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.
- [ ] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [x] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start by locating the validate() functions for blind_transfer_operation, proposal_create_operation, custom_operation, and balance_claim_operation, then read generic_evaluator::prepare_fee(). Compare how each handles negative fee amounts and determine the consistent validation expected by the issue. Done means proposal-wrapped operations cannot carry a negative fee and the affected validation behavior is covered by the project's relevant tests.

Written by the indexing model from the issue text.