LT-14877: ParaTExt folder not found; FLEx quits

When the ParaTExt folder is not found, and FLEx tried to access
ParaTExt, FLEx quit without warning.  This commit also fixes the
"Windows Registry" on Unix so we can read Paratext entries (even
though the Paratext library aparently still can't).

Changes:
Copy PT Unix registry into FW registry before reading.
Check if PT projects folder exists; if not, don't invoke PT.

Change-Id: I4cf96928f9d9436613bf5d22dee83b28be5aaa45