Bubble up AWS error, if creating a new S3Client fails.

Return the error message from `goamz`, rather than crashing.

#### Before

    $ ./dogestry push s3://<bucket> busybox 
    Note: no config file found, using default config.
    Using docker endpoint: tcp://localhost:2375
    WorkDir: /var/folders/l8/g5bn82q153d4s747q_18dgbr0000gp/T/dogestry518011908/busybox
    before newS3Client
    before getS3Auth
    panic: runtime error: invalid memory address or nil pointer dereference
    [signal 0xb code=0x1 addr=0x80 pc=0x12494e]

    goroutine 16 [running]:
    runtime.panic(0x3b4b80, 0x61e1e4)
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/panic.c:279 +0xf5
    github.com/crowdmob/goamz/s3.(*S3).Bucket(0x0, 0x0, 0x0, 0xc2080243f0)
        /Users/toffermann/go/src/github.com/crowdmob/goamz/s3/s3.go:101 +0x1e
    github.com/dogestry/dogestry/remote.(*S3Remote).getBucket(0xc2080f0240, 0x100c1b0)
        /Users/toffermann/go/src/github.com/dogestry/dogestry/remote/s3.go:227 +0x41
    github.com/dogestry/dogestry/remote.(*S3Remote).Validate(0xc2080f0240, 0x0, 0x0)
        /Users/toffermann/go/src/github.com/dogestry/dogestry/remote/s3.go:82 +0x4a
    github.com/dogestry/dogestry/remote.NewRemote(0x7fff5fbffb18, 0x1a, 0xc2080243f0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /Users/toffermann/go/src/github.com/dogestry/dogestry/remote/remote.go:78 +0x272
    github.com/dogestry/dogestry/cli.(*DogestryCli).CmdPush(0xc208038160, 0xc20800e020, 0x2, 0x2, 0x0, 0x0)
        /Users/toffermann/go/src/github.com/dogestry/dogestry/cli/push.go:36 +0x36a
    reflect.callMethod(0xc208025860, 0xc2080e1d18)
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/reflect/value.go:761 +0x18b
    reflect.methodValueCall(0xc20800e020, 0x2, 0x2, 0xc208025860, 0xc20800e301)
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/reflect/asm_amd64.s:26 +0x24
    github.com/dogestry/dogestry/cli.(*DogestryCli).RunCmd(0xc208038160, 0xc20800e010, 0x3, 0x3, 0x0, 0x0)
        /Users/toffermann/go/src/github.com/dogestry/dogestry/cli/cli.go:96 +0x25c
    main.main()
        /Users/toffermann/go/src/github.com/dogestry/dogestry/dogestry/main.go:37 +0x430

    goroutine 19 [finalizer wait]:
    runtime.park(0x14ae0, 0x621848, 0x6202a9)
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/proc.c:1369 +0x89
    runtime.parkunlock(0x621848, 0x6202a9)
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/proc.c:1385 +0x3b
    runfinq()
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/mgc0.c:2644 +0xcf
    runtime.goexit()
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/proc.c:1445

    goroutine 17 [syscall]:
    runtime.goexit()
        /usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/proc.c:1445


#### After
    $ ./dogestry push s3://<bucket> busybox 
    Note: no config file found, using default config.
    Using docker endpoint: tcp://localhost:2375
    WorkDir: /var/folders/l8/g5bn82q153d4s747q_18dgbr0000gp/T/dogestry495606771/busybox
    2014/12/22 13:02:01 No valid AWS authentication found: open /Users/toffermann/.aws/credentials: no such file or directory

cc: @didip