Remove panic from crdt.Tree and index.Tree

[JOOHOJANG]

What this PR does / why we need it:

• Remove panic from crdt/tree, index/tree, element-rht and let them return error when it happens
• let json/... handle error
  Which issue(s) this PR fixes:

Related to #497

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

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

[hackerwins]

Thanks for your contribution.

I did 3 additional minor code cleanups below.

A. We can omit the error checking if the interface of return of caller and method are the same and called it at the end.

AS-IS

func (n *TreeNode) Append(newNodes ...*TreeNode) error {
        // ...
	err := n.IndexTreeNode.Append(indexNodes...)
	if err != nil {
		return err
	}

	return nil
}

TO-BE

func (n *TreeNode) Append(newNodes ...*TreeNode) error {
        // ...
	return n.IndexTreeNode.Append(indexNodes...)
}

B. We can shorten it by binding the error checking with the method call.

https://www.digitalocean.com/community/tutorials/handling-errors-in-go#handling-errors

C. I removed panic from buildDescendants, because it has an interface with an error return.

[codecov]

Codecov Report

Merging #570 (1fda3a6) into main (aa2f361) will decrease coverage by 0.61%.
The diff coverage is 28.83%.

@@            Coverage Diff             @@
##             main     #570      +/-   ##
==========================================
- Coverage   51.91%   51.31%   -0.61%     
==========================================
  Files          67       67              
  Lines        6851     6949      +98     
==========================================
+ Hits         3557     3566       +9     
- Misses       2855     2915      +60     
- Partials      439      468      +29     

Impacted Files	Coverage Δ
api/converter/from_pb.go	48.94% <0.00%> (-0.29%)	⬇️
pkg/document/crdt/text.go	57.84% <ø> (ø)
pkg/document/internal_document.go	21.66% <0.00%> (-0.37%)	⬇️
pkg/index/tree.go	47.90% <25.00%> (-2.27%)	⬇️
pkg/document/crdt/tree.go	53.25% <29.62%> (-11.15%)	⬇️
pkg/document/crdt/root.go	70.49% <33.33%> (-5.38%)	⬇️
pkg/document/document.go	45.12% <33.33%> (-2.32%)	⬇️
pkg/document/crdt/element_rht.go	62.85% <50.00%> (+0.53%)	⬆️
pkg/document/crdt/rga_tree_list.go	62.69% <50.00%> (+0.29%)	⬆️
pkg/document/crdt/array.go	32.07% <100.00%> (ø)
... and 2 more