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]

@CatherineH thank you for your contribution! the PR looks good to me.

May I ask you to add the Signed-off-by: Your Name <[email protected]> line to the commit message?
https://github.com/arduino/arduino-builder/blob/master/CONTRIBUTING.md#submitting-a-pull-request

[cmaglie]

Sorry wrong merge.

@CatherineH your PR is not really merged, please provide a new PR with the commit message fixed as suggested above if you can. Thanks and sorry for the mess...

[CatherineH]

Since I can't figure out how to update this PR, here's a new one:

#141