Show detailed OpenSSL error messages

When OpenSSL encounters an error, M2Crypto prints only the "reason string" (given by ERR_reason_error_string()). This string often contains only very generic message. For example, for all certificate verification errors, the string is the same "certificate verify error".

OpenSSL applications (e.g. the command-line utilities from OpenSSL distribution) additionally show "optional text message" as given by ERR_get_error_line_data(). This message contains more detailed error reason, such as for example "certificate revoked".

Added this message (if given by OpenSSL) to the Python error text. Added corresponding test.