Remove duplicate 'Conversion error:' message

Get rid of duplicate 'Conversion error:' prefix (I found this a bit confusing: are they separate errors?), instead opting for just the error text on a new line with the previous line pointing to it.

Before:
```
  Conversion error: Jekyll::Converters::Scss encountered an error converting 'assets/css/main.scss'.
  Conversion error: Invalid CSS after "... text-overflow": expected ";", was ": ellipsis;"
```

After:
```
  Conversion error: Jekyll::Converters::Scss encountered an error while converting 'assets/css/main.scss':
                    Invalid CSS after "... text-overflow": expected ";", was ": ellipsis;"
```