Report SSL error messages from serf

[minfrin]

Add the most detailed underlying crypto library error string to the error stack when the context fails due to an SSL failure.

SSL errors are no longer reduced to "an error has occurred".

This relies on the serf_ssl_error_cb_t callback as provided by serf in https://github.com/apache/serf/pull/9.

Example:

[minfrin@rocky9 subversion]$ svn info https://svn.example.com/svn/example/core/
svn: E170013: Unable to connect to a repository at URL 'https://svn.example.com/svn/example/core'
svn: E120171: TLS: error:0308010C:digital envelope routines::unsupported
svn: E120171: Error running context: An error occurred during SSL communication

[minfrin]

serf_ssl_error_cb_t callback available in serf since r1926972.