Notify the user if a platform is unknown

Tapasco currently does not notify the user if the platform the user requested is unknown. The following example has a typo in the configuration: the platform specified is 'vc708' instead of the supported 'vc709'

[11:17:47 <main: Tapasco$> INFO] Running with configuration: [Configuration @/opt/rootfs/home/ubuntu/tapasco/default.cfg]
Verbose = Some(verbose)
KernelDir = /opt/rootfs/home/ubuntu/tapasco/kernel
CoreDir = /opt/rootfs/home/ubuntu/tapasco/core
ArchDir = /opt/rootfs/home/ubuntu/tapasco/arch
PlatformDir = /opt/rootfs/home/ubuntu/tapasco/platform
Slurm = false
Parallel = false
MaxThreads = unlimited
Jobs = List(ComposeJob([counter x 128],200.0,Vivado,None,Some(List(vc708)),Some(List(Feature(BlueDMA,Map(enabled -> true)))),None))
[11:17:47 <main: Compose$> INFO] all HLS tasks finished successfully, beginning compose run...
[11:17:47 <main: Compose$> INFO] all Compose tasks finished
[11:17:47 <main: Tapasco$> INFO] TaPaSCo finished successfully