Error in build script when no backends are found

[phil-opp]

Fixes #308

Also:

• warn if backend files exist, but are not accessible (e.g. because access is denied).
• improves the formatting of errors and warnings
• print warnings to stderr instead of stdout

Cargo only shows the stderr/stdout of build scripts if they fail. So the new warnings do not clutter the output of successful builds.

Example output when a folder is inaccessible:

   Compiling arrayfire v3.8.0 (/workspaces/arrayfire-rust)
error: failed to run custom build command for `arrayfire v3.8.0 (/workspaces/arrayfire-rust)`

Caused by:
  process didn't exit successfully: `/workspaces/arrayfire-rust/target/debug/build/arrayfire-09f04fa2319aeba4/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-env-changed=AF_PATH

  --- stderr
  WARNING: USE_LIB is defined, but AF_PATH is not found. Trying to find libraries from known default locations.
  WARNING: failed to access `/opt/arrayfire/lib/libafcuda.dll`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libafcuda.dylib`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libafcuda.so`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libafopencl.dll`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libafopencl.dylib`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libafopencl.so`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libaf.dll`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libaf.dylib`: Permission denied (os error 13)
  WARNING: failed to access `/opt/arrayfire/lib/libaf.so`: Permission denied (os error 13)
  ERROR: no arrayfire backends found

[9prady9]

Sorry about the delay. Couple of minor comments. Can you also please look into the Clippy annotations from the build.rs file. Would be great if you address the other two files also. Thank you!

[9prady9]

I have fixed the clippy warnings in separate commit. If you can look into ci job change and let me know if that is even possible re-use. If it isn't, we can go ahead with this merge. Thank you

[phil-opp]

I have fixed the clippy warnings in separate commit.

Thanks! I rebased this PR on top of the latest changes from master.

[9prady9]

looks good, thank you once again @phil-opp