The tlsrpt_strerror function returns a static string describing the error. If the error is not an
internal error (tlsrpt_error_code_is_internal returns 0 in that case), the text describes the C library
function that gave an error and for C library functions that get called from different places within the
TLSRPT library, the calling function is mentioned in the description. The actual error has to be
retrieved from the C library via the strerror function or its altrnatives. The errno value to be passed
into the strerror function must be extracted from the error code via the tlsrpt_errno_from_error_code
function.