Be more verbose in case of signature check failures

[s-l-teichmann]

Also simplify handling of signature keys.

For #358

[JanHoefelmeyer]

LGTM (Download attempt for working signatures unchanged, for signature check failure now also puts out a reason: Signature Verification Error: Invalid signature)

[tschmidtb51]

I came a cross a weird output:

{
          "num": 20,
          "description": "Public OpenPGP Key",
          "messages": [
            {
              "type": 0,
              "text": "No OpenPGP keys loaded."
            },
            {
              "type": 0,
              "text": "1 public OpenPGP key(s) loaded."
            }
          ]
        }
      ]
    }
  ],
  "version": "2.1.1-21-g7eadf00",

This wasn't the case before:

{
          "num": 20,
          "description": "Public OpenPGP Key",
          "messages": [
            {
              "type": 0,
              "text": "2 public OpenPGP key(s) loaded."
            }
          ]
        }
      ]
    }
  ],
  "version": "2.1.1-20-g3590cf1",

[s-l-teichmann]

I came a cross a weird output:

{
          "num": 20,
          "description": "Public OpenPGP Key",
          "messages": [
            {
              "type": 0,
              "text": "No OpenPGP keys loaded."
            },
            {
              "type": 0,
              "text": "1 public OpenPGP key(s) loaded."
            }
          ]
        }
      ]
    }
  ],
  "version": "2.1.1-21-g7eadf00",

This wasn't the case before:

{
          "num": 20,
          "description": "Public OpenPGP Key",
          "messages": [
            {
              "type": 0,
              "text": "2 public OpenPGP key(s) loaded."
            }
          ]
        }
      ]
    }
  ],
  "version": "2.1.1-20-g3590cf1",

Good catch!
Commit 87ab2ed should solve this.

[tschmidtb51]

LGTM and works

[JanHoefelmeyer]

LGTM