fix: BatchError.Add() non thread safe

[ch3nnn]

BatchError.Add() non thread safe, append operation need locked.

help to review it. thank!

@kevwan @kesonan

[ch3nnn]

example:

fx.Parallel(
	func() {
		_, err := l.svcCtx.xxxRpc.SelectXxx(l.ctx, &Req{})
		if err != nil {
			batchErr.Add(err)
		}
	},
	func() {
		_, err := l.svcCtx.xxxRpc.SelectXxx(l.ctx, &Req{})
		if err != nil {
			batchErr.Add(err)
		}
	},
        ...
        ...
        ...
	func() {
		_, err := l.svcCtx.xxxRpc.SelectXxx(l.ctx, &Req{})
		if err != nil {
			batchErr.Add(err)
		}
	},
)
	
if batchErr.NotNil() {
	return nil, batchErr.Err()
}

batchErr.Err() error message may be incomplete during concurrency

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.53%. Comparing base (5263805) to head (cd351dd).

Additional details and impacted files

Files	Coverage Δ
core/errorx/batcherror.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes