Improve error messages on registry creation

[mssola]

Registry creation is probably the trickiest part of setting up Portus. People deploying with proxies fall into quite some common pitfalls in which Portus reports a puzzling SSL generic error.

We should improve this as soon as possible, since it's affecting the user experience on the very setup of Portus (and sometimes even scaring away users!). Some ideas:

• Try harder on reporting errors:
  • More detailed errors (e.g. trying to detect proxying)
  • With hints to the documentation (e.g. "It seems you have an SSL error, checkout this documentation page for some common pitfalls).
• Update the documentation with common pitfalls.
• Add more examples showcasing stuff like the external_hostname

See #1660