fix: clarify error message when reporting debugger missing

[adrienthebo]

When running cargo-debug with the default debugger (gdb) absent a panic will be generated:

❮ cargo debug build -- -- pubchem view data 13360
args: ["/Users/user/.cargo/bin/cargo-debug", "debug", "build", "--", "--", "pubchem", "view", "data", "13360"]
    Finished dev [unoptimized + debuginfo] target(s) in 0.12s
03:16:12 [INFO] selected binary: "/Users/user/personal/projects/histamix/target/debug/histamix"
thread 'main' panicked at src/main.rs:275:24:
error running debug command: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The panic message doesn't clear indicate that it's the debugger binary that's absent.

This commit adds a bit more error handling to the debugger execution to indicate the cause of the error and suggest a remediation.