caddytls: Give a better error message when given encrypted private keys

[francislavoie]

A user recently pointed out that they were misled because they tried to use an encrypted private key, which we don't support (see #4873).

I chose to read the first 40 characters of the PEM to see if it has ENCRYPTED to guess whether it was encrypted. I chose 40 because the length of the ASCII armor -----BEGIN ENCRYPTED PRIVATE KEY----- is 37, just a rough estimate of the input. I don't know how EC encrypted keys look but I assume it also uses ENCRYPTED in the header?

Before this, the error was:

Error: loading initial config: loading new config: loading http app module: provision http: getting tls app: loading tls app module: provision tls: loading certificates: tls: failed to parse private key

Now:

Error: loading initial config: loading new config: loading http app module: provision http: getting tls app: loading tls app module: provision tls: loading certificates: encrypted private keys are not supported; please decrypt the key first

[mholt]

Nice idea -- thanks!