fix(compile): better error message invoke error

If the command used to compile cannot be found, we now give a sensible
error message, and checks the exit code of the child.

src/commands/compile.zig

@@ -123,7 +123,17 @@ fn compileNote(
     const ext = note.getExtension();
 
     const compiler = try self.getCompiler(allocator, root, ext);
-    return try compiler.compileNote(allocator, note, root, .{});
+    return compiler.compileNote(allocator, note, root, .{}) catch |err|
+        switch (err) {
+        error.FileNotFound => {
+            return cli.throwError(
+                err,
+                "No such command: '{s}'",
+                .{compiler.command[0]},
+            );
+        },
+        else => return err,
+    };
 }
 
 fn getCompiler(

src/topology/TextCompiler.zig

@@ -342,7 +342,9 @@ fn runCommand(
 
     const term = try proc.wait();
     switch (term) {
-        .Exited => {},
+        .Exited => |code| {
+            if (code != 0) return Error.CompileError;
+        },
         .Signal => return Error.CompileInterrupted,
         else => return Error.CompileError,
     }