Fix fetch error handling

[k1LoW]

2019-09-25T10:32:39.957+0900    ERROR    Fetch error    {"host": "example.com", "path": "/var/log/syslog*", "error": "sshkeys: Invalid Passphrase"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x4d9bc9a]

goroutine 69 [running]:
github.com/k1LoW/harvest/collector.(*Collector).Fetch(0x0, 0xc0000da960, 0xc0000d8060, 0xc0000d8080, 0x0, 0x0, 0x0)
    /Users/k1low/src/github.com/k1LoW/harvest/collector/collector.go:93 +0x5a
github.com/k1LoW/harvest/cmd/hrv/cmd.glob..func5.1(0xc000452060, 0xc0005a0800, 0x5377a60, 0xc000430300, 0xc00027d2c0, 0xc0004c57a0, 0xc0000d8060, 0xc0000d8080, 0xc000444b40)
    /Users/k1low/src/github.com/k1LoW/harvest/cmd/hrv/cmd/fetch.go:128 +0x10e
created by github.com/k1LoW/harvest/cmd/hrv/cmd.glob..func5
    /Users/k1low/src/github.com/k1LoW/harvest/cmd/hrv/cmd/fetch.go:121 +0x74b