panic when btcd connection fails and chainSvr is nil

For example, when an invalid CA file is passed:

    btcwallet --simnet --cafile=abc

and the file is not found, `chainSvr` is `nil`, resulting in a panic

    16:27:51 2014-10-14 [ERR] BTCW: Cannot open CA file: open abc: no such file or directory
    16:27:51 2014-10-14 [INF] BTCW: Opened wallet files
    panic: runtime error: invalid memory address or nil pointer dereference
            panic: runtime error: invalid memory address or nil pointer dereference
    [signal 0xb code=0x1 addr=0x1 pc=0x806cb13]

    goroutine 56 [running]:
    runtime.panic(0x84ca040, 0x88d2d93)
            /usr/local/go/src/pkg/runtime/panic.c:279 +0xe9
    main.func·017()
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcwallet/wallet.go:449 +0x33
    runtime.panic(0x84ca040, 0x88d2d93)
            /usr/local/go/src/pkg/runtime/panic.c:248 +0x176
    main.(*Wallet).syncWithChain(0x193bad80, 0x0, 0x0)
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcwallet/wallet.go:464 +0x172
    main.func·016()
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcwallet/wallet.go:373 +0x30
    created by main.(*Wallet).Start
            /home/tuxcanfly/Work/conformal/src/github.com/conformal/btcwallet/wallet.go:377 +0x213

Since the only code being run is when ok is true, I'd move the Start to inside an if ok { } block.  The comment should then mention about only starting the wallet goroutines when the chain rpc client could be created.  In any other case (where creating the client failed), comment that it doesn't matter that the wallet goroutines were never started since the entire program will exit.