SSLDomain raises SSLException with no more information

When methods fail in the proton.SSLDomain class, it's not possible to get more debugging information to the user about what went wrong.

Sample code:

from proton import SSLDomain

ssl = SSLDomain(SSLDomain.MODE_CLIENT)
ssl.set_credentials('/home/user/user.crt', '/home/user/user.key', None)

If "user.crt" or "user.key" are unreadable (ie bad filesystem permissions, or SELinux denials, etc), then set_credentials() fails with a simple error:

proton._exceptions.SSLException: SSL failure.

There's nothing like ENOENT, EPERM, or anything else from openssl, or any other information.

Looking at pn_ssl_domain_set_credentials() in c/src/ssl/openssl.c, two ideas occur to me:

1. This method already logs helpful messages with ssl_log_error(). Is there any way to access those messages with Python?
2. This method returns bare ints for errors (-3, -4, etc). Is there any code to translate those error numbers into other values in Python? Would I have to write that myself? Where?
3. It would be really helpful to get the exact error message from the OpenSSL methods, like when SSL_CTX_use_PrivateKey_file goes wrong, it's hard to tell why.