[dslx:fe] Error message should be better when we backtrack due to e.g. symbol resolution error

**Describe the bug**

With this snippet:

```dslx
fn f() -> u32 { X + Y }

fn main() {
f();
}
```

you get: `Expected '}', got ','` because we backtrack from the symbol resolution error to the `<` and treat that as a less-than operator and then expect the end of the function after the body expression.

If you put the literal in braces:

```dslx
fn f() -> u32 { X + Y }

fn main() {
f<{u32:42}, DOES_NOT_EXIST>();
}
```

you get: `Expected start of an expression; got: {`

Presumably because we don't accept a block expression in the binary right hand side position (although that surprises me as well).

I can only decipher what's happening here because I know the backtracking pops up and goes straight into the "this must be a comparison operator" attempt. I think there's some easier things we can do to make clearer what's happening and some harder ones, but in this case I think we ideally want the "DOES_NOT_EXIST symbol could not be resolved" error to be the propagated, even if we attempt to parse down both backtracking paths.