No error message returned to the user when wrong flag is provided

[davrodpin]

Affected Version

mole compiled from a5252a4

go-env

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="$HOME/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="$HOME/dev/gowork"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/k1/lbj19n2j2ms3xhwx4g30hv480000gp/T/go-build820219202=/tmp/go-build -gno-record-gcc-switches -fno-common"

Platform:

$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.13.6
BuildVersion:	17G65

Expected Behavior

The app should provide the reason for failing its execution when the user is providing the wrong flags or combination of flags

Current Behavior

The app is showing the usage with no error message, as follows:

$ mole -remote :443
usage:
  mole [-v] [-local [<host>]:<port>] -remote [<host>]:<port> -server [<user>@]<host>[:<port>] [-key <key_path>]
  mole -alias <alias_name> [-v] [-local [<host>]:<port>] -remote [<host>]:<port> -server [<user>@]<host>[:<port>] [-key <key_path>]
  mole -alias <alias_name> -delete
  mole -start <alias_name>
  mole -help
  mole -version

  -alias string
        Create a tunnel alias
  -delete
        delete a tunnel alias (must be used with -alias)
  -help
        list all options available
  -key string
        (optional) Set server authentication key file path
  -local value
        (optional) Set local endpoint address: [<host>]:<port>
  -remote value
        set remote endpoint address: [<host>]:<port>
  -server value
        set server address: [<user>@]<host>[:<port>]
  -start string
        Start a tunnel using a given alias
  -v    (optional) Increase log verbosity
  -version
        display the mole version