Prevent accidental portable installation into main installation

[helgoboss]

If the user picks "Portable installation" in ReaBoot but chooses a directory that contains the REAPER executable but is NOT a portable installation (doesn't contain a reaper.ini), installation gets messed up.

This case is not uncommon: If the user installed a non-portable REAPER to a non-default location, picking "Main REAPER installation" will attempt to install REAPER (which should be improved as well). The natural next step for the user is to pick his non-default installation directory as portable installation.

The error message could be:

"This doesn't look like a portable REAPER installation! Maybe this is your main installation and you just installed it in a non-default directory? In this case, please pick 'Main REAPER installation' instead. If not, please pick an empty directory (for creating a new portable installation) or pick an existing portable REAPER installation (a directory that contains a reaper.ini file)!"