ppx_fields_conv: Embed errors in the AST instead of raising

[Burnleydev1]

Coses #10

Hello @panglesd here is a draft pr for the issue, It still has errors which I was talking about in the issue.

[panglesd]

In the metaquot issue, the error happened at a point where it could be directly turned into an AST node: the function was outputing an expression, so you could directly embed the error in an expression and return this.

Here, the errors are checked in a function, which:

• does nothing if everything goes well (returns unit)
• raises otherwise.

So the return type of the function has to be changed! For instance, it could return a result type, as explained for instance in realworldofocaml. In this case, the function could simply return the list of errors. At the call site of the function, if it returns an empty list, continue as usual, otherwise embed the errors in the relevant AST node!

[Burnleydev1]

Thank you @panglesd I get on this right away.