Display help message when fluent arg was referenced incorrectly

The fluent argument syntax is a little special and easy to get wrong, so
we emit a small help message when someone gets it wrong.

Example:
```
parser_mismatched_closing_delimiter = mismatched closing delimiter: `${delimiter}`
```
panics with
```
thread 'rustc' panicked at 'Encountered errors while formatting message for `parser_mismatched_closing_delimiter`
help: Argument `delimiter` exists but was not referenced correctly. Try using `{$delimiter}` instead
attr: `None`
args: `FluentArgs([("delimiter", String("}"))])`
errors: `[ResolverError(Reference(Message { id: "delimiter", attribute: None }))]`', compiler/rustc_errors/src/translation.rs:123:21
```

0 files changed, 0 insertions, 0 deletions