[bootstrap] Give a better error when trying to run a path with no registered step

Before:
```
thread 'main' panicked at 'error: no rules matched invalid', src/bootstrap/builder.rs:287:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

After:
```
error: no `check` rules matched 'invalid'
help: run `x.py check --help --verbose` to show a list of available paths
note: if you are adding a new Step to bootstrap itself, make sure you register it with `describe!`
```

src/bootstrap/builder.rs

@@ -284,7 +284,19 @@ fn run(v: &[StepDescription], builder: &Builder<'_>, paths: &[PathBuf]) {
             }
 
             if !attempted_run {
-                panic!("error: no rules matched {}", path.display());
+                eprintln!(
+                    "error: no `{}` rules matched '{}'",
+                    builder.kind.as_str(),
+                    path.display()
+                );
+                eprintln!(
+                    "help: run `x.py {} --help --verbose` to show a list of available paths",
+                    builder.kind.as_str()
+                );
+                eprintln!(
+                    "note: if you are adding a new Step to bootstrap itself, make sure you register it with `describe!`"
+                );
+                std::process::exit(1);
             }
         }
     }