Value error when loading file with invalid but common underline style element
I have an `.xlsx` file that contains `<u val="1"/>` in the embedded stylesheet `xl/styles.xml`. This invalid value was written by the RubyXL library, due to a bug: https://github.com/weshatheleopard/rubyXL/issues/405
Here is the file: [after_rubyxl_parse_write.xlsx](/uploads/28906062e2eefca06eea201c66d93fd6/after_rubyxl_parse_write.xlsx)
`openpyxl` raises `ValueError` when loading this file:
```
openpyxl.load_workbook("after_rubyxl_parse_write.xlsx")
Traceback (most recent call last):
... (stack trace omitted) ...
ValueError: Value must be one of {'singleAccounting', 'single', 'double', 'doubleAccounting'}
```
Excel loads the file happily and displays a single underline style in the cell. It also does the same for `<u val="total nonsense value I hacked into here"/>`.
While this specific issue is caused by a bug in RubyXL, it looks like invalid values here may be more widespread: a [post on StackOverflow](https://stackoverflow.com/questions/46478902/valueerror-value-must-be-one-of-doubleaccounting-double-singleaccountin/70032017#70032017) suggests that Excel 2016 may also write some invalid underline styles.
Could `openpyxl` be changed to interpret any invalid value of `u/@val` as `single`, to match Excel's behaviour?
Another couple of thoughts:
1. [OpenXML documentation](https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.underlinevalues?view=openxml-2.8.1) suggests that `none` is also a valid value, but it's not listed in the `ValueError`. `none` should be allowed, and interpreted as no underscore (same as absence of `<u>` element).
2. It would be nice if the `ValueError` message also stated the element name, the invalid attribute value and the embedded XML file. That would have made it easier for me to debug the problem.
issue