pkg/mgrconfig: report line and column on parse config error

[Slava0135]

Is your feature request related to a problem? Please describe.

Example error message when there is error in syz manager cfg file:

failed to parse config file: invalid character ']' looking for beginning of value

Not very informative, could be better.

Describe the solution you'd like

Something like this:

failed to parse config file: invalid character ']' looking for beginning of value at <line>:<col>
***also source line maybe***

[a-nogikh]

That would be nice, indeed.

But it would not be a one-line fix unfortunately.

1. We remove comments before parsing the JSON, which must be accounted for
   

   syzkaller/pkg/config/config.go

   Lines 28 to 29 in 77908e5

   	// Remove comment lines starting with #.
   	data = regexp.MustCompile(`(^|\n)\s*#[^\n]*`).ReplaceAll(data, nil)

2. Go's json library only points at the offset from the beginning of the input data, it doesn't care about line numbers
   https://pkg.go.dev/encoding/json#SyntaxError

[Slava0135]

Maybe consider using hjson format (it has comment support) and it also accepts normal json.
Haven't used it though, not sure if their error information is better.

https://github.com/hjson/hjson-go

[dvyukov]

Or perhaps YAML.