Do not allow default ErrorHandler to be set as the ErrorHandler

[MrAlias]

When testing, it is a common need to change the default error handler. It follows that the original should be restored to not affect other code using that default behavior.

func Test(t *testing.T) {
	orig := otel.GetErrorHandler()
	var got error
	handler := otel.ErrorHandlerFunc(func(err error) { got = err })
	otel.SetErrorHandler(handler)
	t.Cleanup(func() { otel.SetErrorHandler(orig) })
	/* ... */
}

Doing this results in the original being set as its own dependent:

runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0xc0206e0388 stack=[0xc0206e0000, 0xc0406e0000]
fatal error: stack overflow

runtime stack:
runtime.throw({0x642212?, 0x1?})
	/usr/lib/go/src/runtime/panic.go:1023 +0x5c fp=0x782b1f3ffca0 sp=0x782b1f3ffc70 pc=0x43b8bc
runtime.newstack()
	/usr/lib/go/src/runtime/stack.go:1103 +0x5bd fp=0x782b1f3ffe50 sp=0x782b1f3ffca0 pc=0x45783d
runtime.morestack()
	/usr/lib/go/src/runtime/asm_amd64.s:616 +0x7a fp=0x782b1f3ffe58 sp=0x782b1f3ffe50 pc=0x472eda

goroutine 200 gp=0xc000102e00 m=9 mp=0xc0001b0008 [running]:
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0xc00011a090?, {0x6a9520?, 0xc000612520?})
	/home/tyler/go/src/go.opentelemetry.io/otel/internal/global/handler.go:36 +0x45 fp=0xc0206e0398 sp=0xc0206e0390 pc=0x595aa5
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0x0?, {0x6a9520?, 0xc000612520?})
	/home/tyler/go/src/go.opentelemetry.io/otel/internal/global/handler.go:37 +0x28 fp=0xc0206e03c0 sp=0xc0206e0398 pc=0x595a88
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0x0?, {0x6a9520?, 0xc000612520?})
	/home/tyler/go/src/go.opentelemetry.io/otel/internal/global/handler.go:37 +0x28 fp=0xc0206e03e8 sp=0xc0206e03c0 pc=0x595a88
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0x0?, {0x6a9520?, 0xc000612520?})
	/home/tyler/go/src/go.opentelemetry.io/otel/internal/global/handler.go:37 +0x28 fp=0xc0206e0410 sp=0xc0206e03e8 pc=0x595a88
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0x0?, {0x6a9520?, 0xc000612520?})
	/home/tyler/go/src/go.opentelemetry.io/otel/internal/global/handler.go:37 +0x28 fp=0xc0206e0438 sp=0xc0206e0410 pc=0x595a88
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0x0?, {0x6a9520?, 0xc000612520?})
	/home/tyler/go/src/go.opentelemetry.io/otel/internal/global/handler.go:37 +0x28 fp=0xc0206e0460 sp=0xc0206e0438 pc=0x595a88
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0x0?, {0x6a9520?, 0xc000612520?})
	/home/tyler/go/src/go.opentelemetry.io/otel/internal/global/handler.go:37 +0x28 fp=0xc0206e0488 sp=0xc0206e0460 pc=0x595a88
go.opentelemetry.io/otel/internal/global.(*ErrDelegator).Handle(0x0?, {0x6a9520?, 0xc000612520?})
[...]

Similar to the other global patterns, the error handler should check and ensure the default is not set as a dependent.