Convert regular exceptions into ExpressionError

I currently try to run validation of SPDX expression from arbitrary inputs. If something is wrong with the expression, I would have expected it to always populate the `errors` attribute instead of raising exceptions. Apparently, this is not the case in every situation.

Let's consider the following snippet:

```python3
from license_expression import get_spdx_licensing

licensing = get_spdx_licensing()
expression = '(Apache-2.0) AND ()'
result = licensing.validate(expression)
print(result.errors)
```

This fails with an *IndexError* during validation:

```
[...]
File "/home/user/lib/python3.9/site-packages/license_expression/__init__.py", line 778, in validate
parsed_expression = self.parse(expression, strict=strict)
File "/home/user/lib/python3.9/site-packages/license_expression/__init__.py", line 545, in parse
expression = super(Licensing, self).parse(tokens)
File "/home/user/lib/python3.9/site-packages/boolean/boolean.py", line 316, in parse
ast[0].append(ast[2])
IndexError: list index out of range
```