Make missing plugin repo errors more informative

[dmytrovoytko]

Thank you for this great editor!
Please consider this update to handle missing plugin repo errors better.
It addresses #1276

Plugins in official channel are ok now (may change someday), but 3 plugins of unofficial channel are missing and it shows up as strange messages while running micro -plugin available or other plugin commands:

Failed to decode repository data:
 json: cannot unmarshal number into Go value of type config.PluginPackages
Failed to decode repository data:
 invalid character '<' looking for beginning of value
Failed to decode repository data:
 invalid character '<' looking for beginning of value

Instead it would show up like this:

Skipped: Plugin repository not found at https://raw.githubusercontent.com/cadnza/mxc/main/repo.json (Status Code: 404).
Skipped: Plugin repository not found at https://notabug.org/dustdfg/micro-calc/raw/master/repo.json (Status Code: 404).
Skipped: Plugin repository not found at https://notabug.org/dustdfg/micro-mdtree/raw/master/repo.json (Status Code: 404).