Improve exceptions

[fkromer]

We should add docs about when which exceptions are raised. ATM there is only one exception FormulaError. It is raised in case of:

• an formula is empty: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L306
• unexpected chars: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L311 , https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L327, https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L351
• unknown symbol: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L359
• missing opening parenthesis: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L392
• missing closing parenthesis: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L332
• count is zero: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L348
• unknown isotope: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L371
• number preceding formula: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L389
• invalid formula: https://github.com/cgohlke/molmass/blob/242b3f410bf2c92e89011442f996cdf9ed430780/molmass/molmass.py#L396

We could improve the docs. From a lib client perspective it would be valuable to categorize or even define specific exceptions to improve error handling capabilities.