Error thrown when Invalid license key character provided

Tern uses license-expression to validate SPDX licenses. When an invalid license key is provided (i.e. contains invalid characters like `/` or `,`), license-expression throws an error when it should handle it.

```
>>> import license_expression
>>> from license_expression import get_spdx_licensing
>>> licensing = get_spdx_licensing()
>>> license_data = "MIT/X11"
>>> licensing.validate(license_data).errors == []

Traceback (most recent call last):
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 777, in validate
parsed_expression = self.parse(expression, strict=strict)
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 539, in parse
tokens = list(self.tokenize(
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 603, in tokenize
for token in tokens:
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 996, in replace_with_subexpression_by_license_symbol
for token_group in token_groups:
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 935, in build_token_groups_for_with_subexpression
tokens = list(tokens)
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 597, in
tokens = (t for t in tokens if t.string and t.string.strip())
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 921, in build_symbols_from_unknown_tokens
for symtok in build_token_with_symbol():
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 901, in build_token_with_symbol
toksym = LicenseSymbol(string)
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 1213, in __init__
raise ExpressionError(
license_expression.ExpressionError: Invalid license key: the valid characters are: letters and numbers, underscore, dot, colon or hyphen signs and spaces: 'MIT/X11'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "", line 1, in
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 780, in validate
expression_info.invalid_symbols.append(e.token_string)
AttributeError: 'ExpressionError' object has no attribute 'token_string'
>>> license_data = "MIT,X11"
>>> licensing.validate(license_data).errors == []
Traceback (most recent call last):
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 777, in validate
parsed_expression = self.parse(expression, strict=strict)
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 539, in parse
tokens = list(self.tokenize(
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 603, in tokenize
for token in tokens:
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 996, in replace_with_subexpression_by_license_symbol
for token_group in token_groups:
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 935, in build_token_groups_for_with_subexpression
tokens = list(tokens)
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 597, in
tokens = (t for t in tokens if t.string and t.string.strip())
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 921, in build_symbols_from_unknown_tokens
for symtok in build_token_with_symbol():
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 901, in build_token_with_symbol
toksym = LicenseSymbol(string)
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 1213, in __init__
raise ExpressionError(
license_expression.ExpressionError: Invalid license key: the valid characters are: letters and numbers, underscore, dot, colon or hyphen signs and spaces: 'MIT,X11'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "", line 1, in
File "/home/rose/ternenv/lib/python3.10/site-packages/license_expression/__init__.py", line 780, in validate
expression_info.invalid_symbols.append(e.token_string)
AttributeError: 'ExpressionError' object has no attribute 'token_string'

```

When a valid license key is provided (i.e. no unexpected characters), the library returns as expected:

```
>>> license_data = "MIT-X11"
>>> licensing.validate(license_data).errors == []
False
```

I would expect the library to handle unexpected characters and mark expressions with unexpected characters as an invalid license.