newt - Fix crash for unrecognized commands.
newt - Fix crash for unrecognized commands.

Example:

[ccollins@ccollins-mac:~/go/src/mynewt.apache.org/newt/newt]$ newt asdas
Error: unknown command "asdas" for "newt"
Run 'newt --help' for usage.
panic: interface conversion: error is *errors.errorString, not *util.NewtError

goroutine 1 [running]:
panic(0x5380e0, 0xc820238640)
        /usr/local/Cellar/go/1.6/libexec/src/runtime/panic.go:464 +0x3e6
mynewt.apache.org/newt/newt/cli.NewtUsage(0x0, 0xd40000, 0xc820236080)
        /Users/ccollins/go/src/mynewt.apache.org/newt/newt/cli/util.go:45 +0x4a7
main.main()
        /Users/ccollins/go/src/mynewt.apache.org/newt/newt/newt.go:150 +0x2db

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt