As Mike Nakis said, echo off only prevents the printing of commands, not results. To hide the result of a command add >nul to the end of the line, and to hide errors add 2>nul. For example:
Del /Q *.tmp >nul 2>nul
Like Krister Andersson said, the reason you get an error is your variable is expanding with spaces:
set INSTALL_PATH=C:\My App\Installer
if exist %INSTALL_PATH% (
Becomes:
if exist C:\My App\Installer (
Which means:
If "C:\My" exists, run "App\Installer" with "(" as the command line argument.
You see the error because you have no folder named "App". Put quotes around the path to prevent this splitting.