Problem with exception symbols when using `get_spdx_licensing().validate()`

Hi, I am trying to validate a given `LicenseExpression` using `get_spdx_licensing().validate()`. This is very helpful in providing a list of unknown symbols not on the SPDX License and Exception Lists. I encountered the problem, though, that exception symbols are also compared against the SPDX License List and licenses against the Exception list:

Example:
```
licensing = get_spdx_licensing()
le = licensing.parse("389-exception with MIT")
get_spdx_licensing().validate(le)
```
yields:
```
ExpressionInfo(
original_expression='389-exception WITH MIT',
normalized_expression='389-exception WITH MIT',
errors=[],
invalid_symbols=[]
)
```
As 389-exception is not a license and MIT not an exception, I would expect an error here. Furthermore, I would find it helpful if there were two separate lists for `invalid_symbols`: For example, one `invalid_license_symbols` and one `invalid_exception_symbols`.