Some cargo errors are not reported in LF

[cmnrd]

A simple way to reproduce this:

1. mkdir -p test/Rust/src-gen/foo
2. lfc test/Rust/src/Minimal.lf

This prints:

/bin/lfc test/Rust/src/Minimal.lf 
Generating code for: file:/home/cmenard/projects/lingua-franca/test/Rust/src/Minimal.lf
******** mode: STANDALONE
******** source file: /home/cmenard/projects/lingua-franca/test/Rust/src/Minimal.lf
******** generated sources: /home/cmenard/projects/lingua-franca/test/Rust/src-gen/Minimal
--- Current working directory: /home/cmenard/projects/lingua-franca/test/Rust/src-gen/Minimal
--- Executing command: cargo +nightly build --out-dir /home/cmenard/projects/lingua-franca/test/Rust/bin -Z unstable-options --message-format json-diagnostic-rendered-ansi
lfc: error: cargo failed with error code 101
 --> (unknown file):null:1 - cargo failed with error code 101

lfc: fatal error: Aborting due to previous error

I have only tested this with lfc, but I suspect that also the IDE will not show a useful error message here.

The error message reported by cargo in this case is:

error: failed to load manifest for workspace member `/home/cmenard/projects/lingua-franca/test/Rust/src-gen/foo`

Caused by:
  failed to read `/home/cmenard/projects/lingua-franca/test/Rust/src-gen/foo/Cargo.toml`

Caused by:
  No such file or directory (os error 2)