fix: Improve protoc not found error message

[allan2]

• fixes missing whitespace between the error message and the OS-specific hint
• more succinct wording in error message
• capitalization ("debian" -> "Debian")
• consistent grammar tenses, slightly more formal and direct tone
• os_specific_hint now has hard breaks at 80 chars (just like error_msg)

Before:

 --- stderr
  thread 'main' panicked at /home/allan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/prost-build-0.12.1/src/lib.rs:1475:10:
  Could not find `protoc` installation and this build crate cannot proceed without
      this knowledge. If `protoc` is installed and this crate had trouble finding
      it, you can set the `PROTOC` environment variable with the specific path to your
      installed `protoc` binary.If you're on debian, try `apt-get install protobuf-compiler` or download it from https://github.com/protocolbuffers/protobuf/releases

  For more information: https://docs.rs/prost-build/#sourcing-protoc

Proposed:

 --- stderr
  thread 'main' panicked at tests/single-include/build.rs:7:10:
  called `Result::unwrap()` on an `Err` value: Custom { kind: NotFound, error: "Could not find `protoc`. If `protoc` is installed, try setting the `PROTOC` environment variable to the path of the `protoc` binary. To install it on Debian, run `apt-get install protobuf-compiler`. It is also available at https://github.com/protocolbuffers/protobuf/releases  For more information: https://docs.rs/prost-build/#sourcing-protoc" }

One difference is that the previous message has a hanging indent, but this PR removes it. I would be happy to add it back.

[caspermeijn]

Please rebase to latest master to fix CI

[caspermeijn]

Please rebase to latest master to fix CI

I see you rebase, but I just merged another PR related to protoc not found. Can you rebase again? Thanks

[allan2]

Rebased on top of latest.

[caspermeijn]

I have replaced the new lines by spaces. Due to how unwrap works, the new lines are not displayed anymore

[caspermeijn]

Thank you for your contribution