Unified error handling

[olemis]

Feature description

Singleton values to represent error conditions .

Is your feature request related to a problem? Please describe.
Inline error objects defined across source code

$ grep -nr 'errors.New' src/
src//core/network.go:78:			return nil, errors.New("There is not available objects")
src//core/network.go:97:		return errors.New(fmt.Sprintf("There is not exist %s poolSection", poolSection))
src//core/network.go:103:		return errors.New(fmt.Sprintf("That object is no from this pool"))
src//util/pluginutil.go:20:	return uint64(0), errors.New(ticker + " <Unregistered>")
Binary file src//coin/skycoin/models/.wallet.go.swp matches
src//coin/skycoin/models/blockchain.go:19:		return nil, errors.New("Block not setted or nil")
src//coin/skycoin/models/blockchain.go:26:		return nil, errors.New("Block not setted or nil")
src//coin/skycoin/models/blockchain.go:33:		return 0, errors.New("Block not setted or nil")
src//coin/skycoin/models/blockchain.go:40:		return 0, errors.New("Block not setted or nil")
src//coin/skycoin/models/blockchain.go:46:		return 0, errors.New("Block not setted or nil")
src//coin/skycoin/models/blockchain.go:53:		return 0, errors.New("Block not setted or nil")
src//coin/skycoin/models/blockchain.go:63:		return true, errors.New("Block not setted or nil")
src//coin/skycoin/models/blockchain.go:133:			return 0, errors.New("Number of Blocks unset ")
src//coin/skycoin/models/network.go:47:		return nil, errors.New(fmt.Sprintf("There is not propers client in %s pool", section))
src//coin/skycoin/models/wallet.go:238:		return "", errors.New("Entropy must be 128 or 256")
src//coin/skycoin/models/wallet.go:371:	return errors.New("Value errors")
src//main/config.go:74:		return errors.New("Invalid Id")

Describe the solution you'd like
Declare single constant and use it consistently across all (related) packages .