Add specific exceptions for String issues

[ashafer01]

My application can expect either raw bytes or a string in the same underlying ASN1 components. The current exception hierarchy makes this situation basically impossible to handle.

I have added new exceptions specific to string encoding and decoding, and utilized multiple inheritance to ensure that users catching any relevant type of error will still catch these errors. i.e. users that have updated their code to catch PyAsn1Error in this situation can continue doing so unaffected, and users that were already catching the Unicode*Errors and have not updated will be fixed with this patch.

Also worth noting that Python .encode() and .decode() will always use the relevant Unicode*Error regardless of the passed encoding.

[codecov-io]

Codecov Report

Merging #155 into master will decrease coverage by 0.04%.
The diff coverage is 26.08%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
- Coverage   85.64%   85.59%   -0.05%     
==========================================
  Files          29       29              
  Lines        4076     4083       +7     
==========================================
+ Hits         3491     3495       +4     
- Misses        585      588       +3

Impacted Files	Coverage Δ
pyasn1/type/univ.py	83.58% <12.5%> (ø)	⬆️
pyasn1/type/char.py	92.12% <12.5%> (ø)	⬆️
pyasn1/error.py	70% <57.14%> (-30%)	⬇️

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[etingof]

Sorry for long delay...

Alternatively, would it make sense to let unicode-related exceptions to bubble up as-is? I fair that duplicating system exceptions is going to become messy very soon...

[ashafer01]

Yeah I think letting them bubble would be much cleaner and more maintainable, but I wanted to provide a compatible solution since this behavior has changed over various releases.

With that in mind, I think for the spots where LookupError is also handled it would be better and more compatible to throw as a PyAsn1Error as before, but always just re-raise (or just dont catch) UnicodeErrors.

[etingof]

Contemplating this matter more, I think your initial solution makes more sense for now. In the next, less compatible, revisions we could just let UnicodeError to bubble up...

Thank you!

[etingof]

JFYI: I've renamed the exception classes in the follow up patch in hope to make it more obvious to the users.