You don't need to leak abstractions on exceptions. Libraries should wrap the exceptions and provide meaningful exceptions, not dependent on the exact implementation. Example:

  public String setCache(String key, String value) throws CacheUnavailableException,CacheWriteException {
    try {
    ...
    } catch (SocketException e) {
        throw new CacheUnavailableException(e);
    } catch (SomeMemcacheException e) {
        throw new CacheWriteException(e);
    }
  }

This will allow library clients to catch exceptions that mean something to them, not exceptions that only make sense if you know how the library is using memcache.