Make CLI functions return error instead of fatal out (#1379)

Before this PR, CLI functions fatal out when encountering error
instead of returning it. Such behavior makes it hard to reuse 
the functions nor writing unit tests. This PR aims to make functions 
return errors instead of error out.

pkg/fission-cli/flag/const.go

@@ -0,0 +1,56 @@
+/*
+Copyright 2019 The Fission Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package flag
+
+import "strings"
+
+const (
+	GLOBAL_VERBOSITY = "verbosity"
+	GLOBAL_PLUGIN    = "plugin"
+
+	FISSION_SERVER = "server"
+
+	RESOURCE_NAME = "name"
+
+	ENVIRONMENT_NAMESPACE          = "envNamespace"
+	ENVIRONMENT_NAMESPACE_ALIAS    = "envns"
+	ENVIRONMENT_POOLSIZE           = "poolsize"
+	ENVIRONMENT_IMAGE              = "image"
+	ENVIRONMENT_BUILDER            = "builder"
+	ENVIRONMENT_BUILDCOMMAND       = "buildcmd"
+	ENVIRONMENT_KEEPARCHIVE        = "keeparchive"
+	ENVIRONMENT_EXTERNAL_NETWORK   = "externalnetwork"
+	ENVIRONMENT_GRACE_PERIOD       = "graceperiod"
+	ENVIRONMENT_GRACE_PERIOD_ALIAS = "period"
+	ENVIRONMENT_VERSION            = "version"
+
+	SPEC_SPEC    = "spec"
+	SPEC_SPECDIR = "specdir"
+
+	RUNTIME_MINCPU    = "mincpu"
+	RUNTIME_MAXCPU    = "maxcpu"
+	RUNTIME_MINMEMORY = "minmemory"
+	RUNTIME_MAXMEMORY = "maxmemory"
+	RUNTIME_MINSCALE  = "minscale"
+	RUNTIME_MAXSCALE  = "maxscale"
+	RUNTIME_TARGETCPU = "targetcpu"
+)
+
+// GetCliFlagName concatenates flag and its alias into a command flag name.
+func GetCliFlagName(flags ...string) string {
+	return strings.Join(flags, ", ")
+}