More structured errors

[remexre]

It'd be nice to be able to get errors more precisely when #234 / #297 gets resolved. For example,

tera/src/tera.rs

Lines 195 to 199 in 97174b7

	bail!(
	"Circular extend detected for template '{}'. Inheritance chain: `{:?}`",
	start.name,
	parents,
	);

and

tera/src/tera.rs

Lines 208 to 211 in 97174b7

	None => bail!(
	"Template '{}' is inheriting from '{}', which doesn't exist or isn't loaded.",
	template.name, p,
	),

should return their own enum branch.

[Keats]

Can you expand a bit on the errors you want? Both for parsing and rendering stages.

I was planning to have a pretty basic error enum since I didn't think people would want specific errors. I thought that in most of the cases I can think of they would just print them and exit, at least for the parsing stage. In the 2 cases linked, why do you need 2 different enum branch to represent them rather than a simple CoherenceError(String) or something similar?

[remexre]

It'd be nice to at least have the names of the template with the error and the template that's missing/circular, so maybe a CoherenceError(CoherenceErrorKind, PathBuf, PathBuf) would be better?
(where enum CoherenceErrorKind { Circular, Missing }).

[Keats]

But how would you handle differently errors like CircularInheritance, MissingTemplate or MissingMacroFile in your code? You can't really do much other than print the error as it's not fixable from the Rust side

[remexre]

This is for a use-case where I'm loading templates from a database, so I can handle the missing file errors as "check the db for the file and retry if found."

[Keats]

Ah didn't think of that usecase. I'll have separate enum kinds then.
What about rendering? Just a single RenderFailed error?

[remexre]

Nothing in my usecase needs a more specific error there, but it might be useful to be able to point to a span corresponding to the error?

[Keats]

Adding spans to the AST is a significant amount of work that I am probably not going to do myself but I would welcome a PR for it

[Keats]

Errors are now without error-chain in the v1 branch if you want to have a go at it for the errors you need!

[Keats]

@remexre I pushed some new error types on the v1 branch if you want to try it out