Fix fetching issue error handling, improve its errors, validate tracking issues, fix errors in goals

[lqd]

Creating issues is failing right now, with a cryptic error message.

Error: error: failed to adjust issues; rerun command to resume
   --> crates/rust-project-goals-cli/src/main.rs:160:28
    |
160 |                 e.wrap_str(Spanned::here(
    |                            ^^^^^^^^^^^^^^
    |
   ::: /home/lqd/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/spanned-0.6.1/src/error.rs:130:23
    |
130 |                 span: Span::here(),
    |                       ^^^^^^^^^^^^^ EOF while parsing a value at line 1 column 0
    |

The problem was that fetching the issue failed, but the code didn't check for the command status and tried to parse an empty response.

This PR fixes this error handling issue (and there may be more of this pattern elsewhere) and also improves the error to add more information about the goal causing the issue:

Error: error: failed to adjust issues; rerun command to resume
   --> crates/rust-project-goals-cli/src/main.rs:160:28
    |
160 |                 e.wrap_str(Spanned::here(
    |                            ^^^^^^^^^^^^^^
    |
   ::: crates/rust-project-goals-cli/src/rfc.rs:306:36
    |
306 |                         e.wrap_str(Spanned::here(format!(
    |                                    ^^^^^^^^^^^^^^^^^^^^^^ error while fetching declared tracking issue 891 for goal ./src/2025h2/gcc-backend-tests.md
    |
   ::: crates/rust-project-goals/src/gh/issues.rs:116:9
    |
116 |         spanned::bail_here!(
    |         ^^^^^^^^^^^^^^^^^^^^ fetching `rust-lang/rust-project-goals` issue 891 failed: GraphQL: Could not resolve to an issue or pull request with the number of 891. (repository.issue)

    |

It also adds a validation step for the Tracking issue row, to ensure they point to this repository, and suggests using another row when needed:

Error: error: tracking issues are issues in the rust-project-goals repository. The `[rust-lang/compiler-team#891]` issue can go in the `Other tracking issues` row.
   --> src/2025h2/gcc-backend-tests.md:7:22
    |
  7 | | Tracking issue   | [rust-lang/compiler-team#891]                  |
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
   ::: crates/rust-project-goals/src/goal.rs:459:13
    |
459 |             spanned::bail!(
    |             ^^^^^^^^^^^^^^^ error reported here

And finally, it fixes the "Tracking issue" problem in the 3 failing goals.

Rendered