Blender Git Loki

Blenlib: Add BLI_assert_msg() for printing an extra string if the assert fails

It always bothered me that we'd do the `BLI_assert(... || !"message")` trick to
print a message alongside the assert, while it should be trivial to have a way
to pass an extra string as additional argument.

This adds `BLI_assert_msg()` with a second argument for a message. E.g.:
```
BLI_assert_msg(
params->rename_id == NULL,
"File rename handling should immediately clear rename_id when done, because otherwise it will keep taking precedence over renamefile.");
```

On failure this will print like this:
```
0 Blender 0x00000001140647a3 BLI_system_backtrace + 291
[...]
13 Blender 0x00000001092647a6 main + 3814
14 libdyld.dylib 0x00007fff203d8f5d start + 1
BLI_assert failed: source/blender/editors/space_file/file_ops.c:2352, file_directory_new_exec(), at 'params->rename_id == ((void*)0)'
File rename handling should immediately clear rename_id when done, because otherwise it will keep taking precedence over renamefile.
```

Reviewed by: Sybren St�vel, Jacques Lucke, Sergey Sharybin, Campbell Barton

Differential Revision: https://developer.blender.org/D11827