box: add error code for SSLError

Applier uses box_error_code() to for better logging:
 - It remembers tha last raised error code and skips logging if the new
   error code is the same.
 - It logs "will retry every X seconds" only for retryable error codes
   (for example, ER_SYSTEM) while for non-retryable errors (for example,
   ER_PROC_LUA) the message isn't logged.

box_error_code() returns ER_PROC_LUA for SSLError, which is confusing
and would result in inconsistent logging in applier if we made SSLError
retryable. Let's add a separate error code for this error (ER_SSL) and
introduce a test case that checks that box_error_code() works as
expected for all kinds of errors.

Follow-up commit a7028dde ("Add SSL iostream stub").
Needed for https://github.com/tarantool/tarantool-ee/issues/107

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