More detailed error message for InternalError

Uses ERR_error_string_n because the source contains horrible warnings
against ever using ERR_error_string.
Merge pull request #545 from public/error-strings

More detailed error message for InternalError
Only put a single top level header in this page
Merge pull request #546 from alex/cleanup-index

Only put a single top level header in this page
Rename RSAs key_length to key_size

So that it matches the existing documented CipherContext stuff.
Merge pull request #548 from public/rename-key-length

Rename RSAs key_length to key_size