Detailed errors for include_file!

[teh-cmc]

Fixes #1322.

Before:

thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: No such file or directory (os error 2)', crates/re_renderer/src/renderer/mesh_renderer.rs:286:25
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:64:14
   2: core::result::unwrap_failed
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/result.rs:1791:5
   3: core::result::Result<T,E>::unwrap
[...]

After:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: include_file!(../../shader/instanced_mesh.wgsl) (rooted at "crates/re_renderer/src/renderer/mesh_renderer.rs") failed while trying to import physical path "crates/re_renderer/src/renderer/../../shader/instanced_mesh.wgsl"

Caused by:
    No such file or directory (os error 2)', crates/re_renderer/src/renderer/mesh_renderer.rs:286:25
stack backtrace:
   0: rust_begin_unwind
             at /rustc/fc594f15669680fa70d255faec3ca3fb507c3405/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/fc594f15669680fa70d255faec3ca3fb507c3405/library/core/src/panicking.rs:64:14
   2: core::result::unwrap_failed
[...]

[Wumpf]

nice