This PR gives a focused error message when a user tries to name an
example, and tweaks error messages for attempts to define multiple
opaque names at once.
## Example errors
```
example x : 1 == 1 := by grind
```
Current message:
```
Failed to infer type of binder `x`
Note: Because this declaration's type has been explicitly provided, all parameter types and holes (e.g., `_`) in its header are resolved before its body is processed; information from the declaration body cannot be used to infer what these values should be
```
New message:
```
Failed to infer type of binder `x`
Note: Examples don't have names. The identifier `x` is being interpreted as a parameter `(x : _)`.
```
## Plural-aware identifier lists
Both the example errors and opaque errors understand pluralization and
use oxford commas.
```
opaque a b c : Nat
```
Current message:
```
Failed to infer type of binder `c`
Note: Multiple constants cannot be declared in a single declaration. The identifier(s) `b`, `c` are being interpreted as parameters `(b : _)`, `(c : _)`.
```
New message:
```
Failed to infer type of binder `c`
Note: Multiple constants cannot be declared in a single declaration. The identifiers `b` and `c` are being interpreted as parameters `(b : _)` and `(c : _)`.```
This PR updates the formatting of, and adds explanations for, "unknown
identifier" errors as well as "failed to infer type" errors for binders
and definitions.
It attempts to ameliorate some of the confusion encountered in #1592 by
modifying the wording of the "header is elaborated before body is
processed" note and adding further discussion and examples of this
behavior in the corresponding error explanation.
This PR improves the type-as-hole error message. Type-as-hole error for
theorem declarations should not admit the possibility of omitting the
type entirely.
---------
Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
