Improve error handling and message on Config/ACME failures

[mhabegger]

Config or ACME failures result in missing certificates, that DFLE then tries to send to DFP.
Improve the handling on errors and their message and avoid sending empty files to DFP.

As an example, the log from DFLE container when the Domain list is not set properly :

Docker Flow: Proxy DNS-Name: http://proxy
current folder name is:
concat certificates for
/root/renewAndSendToProxy.sh: line 17: cd: /etc/letsencrypt/live/*/: No such file or directory
cat: cert.pem: No such file or directory
cat: chain.pem: No such file or directory
cat: privkey.pem: No such file or directory
generated .combined.pem
transmit .combined.pem to http://proxy
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   123  100   123    0     0  14846      0 --:--:-- --:--:-- --:--:-- 15375
HTTP/1.1 400 Bad Request
Date: Sat, 04 Mar 2017 11:43:57 GMT
Content-Length: 123
Content-Type: text/plain; charset=utf-8

{"Status":"NOK","Message":"Could not send distribute request to the following addresses: [10.0.0.3 10.0.0.4]","Certs":null}proxy received .combined.pem
/etc/letsencrypt will be backed up as backup-date-time.tar.gz. It's important to know that some files are symbolic links (inside this backup) and they need to be untared correctly.
Backup created, if you like download the /etc/letsencrypt/backup folder and store it on a safe place!

Thanks for using Docker Flow: Let's Encrypt and have a nice day!

Starting supervisord (which starts and monitors cron)
2017-03-04 11:43:57,822 CRIT Supervisor running as root (no user in config file)
2017-03-04 11:43:57,824 INFO supervisord started with pid 244
2017-03-04 11:43:58,827 INFO spawned: 'cron' with pid 247
2017-03-04 11:43:59,828 INFO success: cron entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)