Remove panic from crdt.RGATreeList

[sejongk]

What this PR does / why we need it:
This removes panic in crdt/rga_tree_list that should be avoided in production.

Which issue(s) this PR fixes:
Related to #497

Special notes for your reviewer:
I guess it would be good to add some test cases for errors.
Would you mind giving me some comments about this?
I'm just planning to add tests for a basic success and the errors included in this pr.

Does this PR introduce a user-facing change?:

Additional documentation

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #596 (9b3b338) into main (1b557be) will increase coverage by 0.36%.
Report is 4 commits behind head on main.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
+ Coverage   51.11%   51.47%   +0.36%     
==========================================
  Files          66       66              
  Lines        7008     6962      -46     
==========================================
+ Hits         3582     3584       +2     
+ Misses       2952     2904      -48     
  Partials      474      474              

Files Changed	Coverage Δ
api/converter/from_bytes.go	52.42% <0.00%> (-0.75%)	⬇️
pkg/document/crdt/element_rht.go	62.85% <0.00%> (ø)
pkg/document/crdt/object.go	21.62% <ø> (ø)
pkg/document/crdt/array.go	30.35% <33.33%> (-1.72%)	⬇️
pkg/document/crdt/rga_tree_list.go	80.15% <70.00%> (+17.45%)	⬆️

... and 9 files with indirect coverage changes

[hackerwins]

Thanks for your contribution.

Overall it looks good. I left a minor comment related to style.
And if possible, it would be nice to add related test codes as well.

[dc7303]

Could you tell me why panic is called?
I think it's safer to handle errors using errors.

[dc7303]

Oh, are you going to phase it out?

[sejongk]

I thought it is okay to call panic in json according to #497 (comment).
It would be great to change the above logic to handle errors if possible.
I guess one of possible errors can be an error that occurs because of a failure of finding a node for the given createdAt.

func (a *RGATreeList) findNextBeforeExecutedAt(
	createdAt *time.Ticket,
	executedAt *time.Ticket,
) (*RGATreeListNode, error) {
	node, ok := a.nodeMapByCreatedAt[createdAt.Key()]
	if !ok {
		return nil, fmt.Errorf("fail to find the given createdAt: %s", createdAt.Key())
	}

	...
}

Would you mind giving me some guides or examples to safely handle errors like the above?

[hackerwins]

I left one more minor comment.