specs: emit better parsing errors for specs. (#24860)

Parse error information is kept for specs, but it doesn't seem like we propagate it
to the user when we encounter an error.  This fixes that.

e.g., for this error in a package:

```python
    depends_on("python@:3.8", when="0.900:")
```

Before, with no context and no clue that it's even from a particular spec:

```
==> Error: Unexpected token: ':'
```

With this PR:

```
==> Error: Unexpected token: ':'
  Encountered when parsing spec:
    0.900:
         ^
```

0 files changed, 0 insertions, 0 deletions