improve errors location

[eric-therond]

• use of the span.error() function to have a better location of errors
• except for "internal errors" when the end-user is not responsible / can't do anything with the error?

[anakrish]

LGTM

[anakrish]

@eric-therond
Can you fix this error?

error: this if branch is empty
--> src/interpreter.rs:552:25
|
552 | if raise_error {}
| ^^^^^^^^^^^^^^^^^ help: you can remove it
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_if
= note: -D clippy::needless-if implied by -D warnings