Make the output for invalid / no --type parameter more verbose.

[cfi-gb]

Fix coding style inconsistencies for all if[ ]; then cases.

To test run something like e.g. the following two for the output:

greenbone-feed-sync --type foo

greenbone-feed-sync foo

and for the help banner:

greenbone-feed-sync --help

Previous to this PR the output looked like:

No feed type given

and

Invalid feed type FOO

Now we're more verbose / having a more helpful message like:

No feed type given to --type parameter

and

Invalid feed type FOO given to --type parameter. Currently supported: CERT, SCAP or GVMD_DATA

Checklist:

• Tests
• CHANGELOG Entry

[bjoernricks]

👍