Print our own callstack on panics

[emilk]

The panic handler prints very long filenames, which can include sensitive paths. By cutting out the sensitive parts we get two benefits:

• Users can copy-paste the callstack from the terminal into a rerun issue without any sensitive paths leaking
• The callstack is much more readable.

Before:

[2023-03-20T14:51:43Z INFO  rerun::run] Loading "../api_demo.rrd"…
thread 'main' panicked at 'assertion failed: from.start() != from.end()', /Users/emilk/.cargo/registry/src/github.com-1ecc6299db9ec823/emath-0.21.0/src/lib.rs:142:5
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::panicking::panic
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:111:5
   3: emath::rect_transform::RectTransform::transform_pos
   4: emath::rect_transform::RectTransform::transform_rect
             at /Users/emilk/.cargo/registry/src/github.com-1ecc6299db9ec823/emath-0.21.0/src/rect_transform.rs:52:18
   5: re_viewer::ui::view_spatial::ui::create_labels
             at ./crates/re_viewer/src/ui/view_spatial/ui.rs:616:13

After:

[2023-03-20T14:49:46Z INFO  rerun::run] Loading "../api_demo.rrd"…

thread 'main' panicked at 'assertion failed: from.start() != from.end()', emath-0.21.0/src/lib.rs:142

   7: core::panicking::panic_fmt
             at core/src/panicking.rs:64:14
   8: core::panicking::panic
             at core/src/panicking.rs:111:5
   9: emath::rect_transform::RectTransform::transform_pos
      emath::rect_transform::RectTransform::transform_rect
             at emath-0.21.0/src/rect_transform.rs:52:18
  10: re_viewer::ui::view_spatial::ui::create_labels
             at re_viewer/src/ui/view_spatial/ui.rs:616:13
…

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct

[jleibs]

Nit: Doing these trim operations on the string-concatenated callstack after it's been converted to a string still seems a bit awkward with the rfind('\n') business. This seems like it could maybe be more cleanly pushed down into backtrace_to_string as an operation on for frame in backtrace.frames().

[emilk]

I agree its ugly, but the backtrace crate is a bit too opaque (non-pub) to do anything nicer.

[jleibs]

The After version reads so much more nicely. I love it.