resubmit with signed off - added exception for incorrect board name string

[CatherineH]

I just started using arduino-builder a few days ago, and was confused when I got the error:

$ arduino-builder -fqbn teensyLC -hardware ~/arduino-1.6.7/hardware/ -tools ~/arduino-1.6.7/tools experiment_control.ino
panic: runtime error: index out of range

goroutine 1 [running]:
arduino.cc/builder.(*TargetBoardResolver).Run(0x6c3050, 0xc20803c4e0, 0x0, 0x0)
    ~/arduino-builder/src/arduino.cc/builder/target_board_resolver.go:46 +0xf44
arduino.cc/builder.(*ContainerSetupHardwareToolsLibsSketchAndProps).Run(0x6c3050, 0xc20803c4e0, 0x0, 0x0)
    ~/arduino-builder/src/arduino.cc/builder/container_setup.go:59 +0x6b8
arduino.cc/builder.runCommands(0xc20803c4e0, 0xc20802db90, 0x1d, 0x1d, 0x418001, 0x0, 0x0)
    ~/arduino-builder/src/arduino.cc/builder/builder.go:187 +0x139
arduino.cc/builder.(*Builder).Run(0xc20802dd88, 0xc20803c4e0, 0x0, 0x0)
    ~/arduino-builder/src/arduino.cc/builder/builder.go:118 +0xef7
arduino.cc/builder.RunBuilder(0xc20803c4e0, 0x0, 0x0)
    ~/arduino-builder/src/arduino.cc/builder/builder.go:218 +0x49
main.main()
    ~/arduino-builder/main.go:333 +0x1eb2

With this pull request, the new error message is:

$ arduino-builder -fqbn teensyLC -hardware ~/arduino-1.6.7/hardware/ -tools ~/arduino-1.6.7/tools experiment_control.ino
teensyLC is not a valid fully qualified board name. Required format is targetPackageName:targetPlatformName:targetBoardName.

[cmaglie]

I've merged 6a48e71 and skipped all the merge commits. I've also ported the implementation to the new context layout, see b71154d.

Thank you!