Added validation, error handling and some debug messages for credentials

[Kraemii]

What:
Added validation, error handling and some debug messages for credentials

Why:
Validation was missing on the ospd-openvas side. Also added some error messages to know exactly what was wrong.

How:
I tested each validation and error messages and also tested some proper scans with gvm-cli and also GSA, to know that we don't get errors when everything should be fine.
Template script for testing with gvm-cli:

<start_scan scan_id='97079ee9-8917-49da-aa4f-4ef95f757ac1'>
  <vt_selection>
    <vt_single id='1.3.6.1.4.1.25623.1.0.100315'>
      <vt_value id='9'>1</vt_value>
      <vt_value id='6'>1</vt_value>
      <vt_value id='3'>1</vt_value>
    </vt_single>
  </vt_selection>
  <targets>
    <target>
      <hosts>127.0.0.1</hosts>
      <ports>80,8080,443,22</ports>
      <exclude_hosts></exclude_hosts>
      <credentials>
        <credential service="ssh" type="up" port="22">
          <username>user</username>
          <password>pw</password>
        </credential>
      </credentials>
    </target>
  </targets>
  <scanner_params>
  </scanner_params>
</start_scan>

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #416 (ae4ae2d) into master (a77d367) will decrease coverage by 0.11%.
The diff coverage is 79.62%.

@@            Coverage Diff             @@
##           master     #416      +/-   ##
==========================================
- Coverage   84.21%   84.09%   -0.12%     
==========================================
  Files           9        9              
  Lines        1539     1584      +45     
==========================================
+ Hits         1296     1332      +36     
- Misses        243      252       +9     

Impacted Files	Coverage Δ
ospd_openvas/daemon.py	66.78% <0.00%> (-0.75%)	⬇️
ospd_openvas/preferencehandler.py	89.85% <91.48%> (+0.30%)	⬆️

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