Errors in mount_build_disk.sh should stop build if disk not mounted

[Rob Nelson]

If the secondary disk is not mounted by mount_build_disk.sh no further builds will occur

• add additional check to ensure we have the secondary disk online when checking diskspace
  for a build (gitian-build.sh)

• catch errors and fail VBox_VM_Initialize.sh if the build disk is not mounted or other errors occur

[Rob Nelson]

appears to be working correctly in v0.1.0