Msiexec error if DefaultInstallDir does not exist on target machine

[cordifed]

When using the DefaultInstallDir option in btdfproj, and referring to a fixed location on a drive (e.g. E:\ ) that doesn't exist, executing the msi results in an error message:

The folder path '...' contains an invalid character.

See comment in https://stackoverflow.com/questions/48465432/deploy-biztalk-application-msi-created-with-btdf-via-msiexec/48483638#48483638