private/app/launcher: unwrap errors fully to avoid hiding cause

As an example, before this commit, when a key was removed from the
struct that defines available configuration option for the toml file for
a given application such as the scion-dispatcher, it would fail without
reporting what key is present when it should be removed from the config
file, as follows:

error: loading config from file {file=/foo.toml}: strict mode: fields in the document are missing in the target struct

After this commit it will fail more informatively as follows:

error: loading config from file {file=/foo.toml}: strict mode: fields in the document are missing in the target struct &toml.StrictMissingError{Errors:[]t
oml.DecodeError{toml.DecodeError{message:"missing field", line:4, column:1, key:toml.Key{"general", "reconnect_to_dispatcher"}, human:"1| [general]\n2| config_dir = \"/etc/scion\"\n3| id = \"sd\"\n4| reconnect_to_dispatcher = true\n | ~~~~~~~~~~~~~~~~~~~~~
~~ missing field\n5|\n6| [log.console]\n7| level = \"info\""}}}

You can see the reason for the failure is the presence of the "reconnect_to_dispatcher" key, which no longer exists

Author: MatthewCroughan

private/app/launcher/launcher_unix.go

@@ -23,6 +23,7 @@ import (
 	"path/filepath"
 	"syscall"
 	"time"
+	"errors"
 
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
@@ -46,7 +47,7 @@ type Application struct {
 // Run will exit the application if it encounters a fatal error.
 func (a *Application) Run() {
 	if err := a.run(); err != nil {
-		fmt.Fprintf(a.getErrorWriter(), "fatal error: %v\n", err)
+		fmt.Fprintf(a.getErrorWriter(), "fatal error: %v\n %#v\n", err, errors.Unwrap(err))
 		os.Exit(1)
 	}
 }

private/app/launcher/launcher_windows.go

@@ -24,6 +24,7 @@ import (
 	"path/filepath"
 	"syscall"
 	"time"
+	"errors"
 
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
@@ -73,7 +74,7 @@ type Application struct {
 // Run will exit the application if it encounters a fatal error.
 func (a *Application) Run() {
 	if ec, err := a.run(); err != nil {
-		fmt.Fprintf(a.getErrorWriter(), "fatal error: %v\n", err)
+		fmt.Fprintf(a.getErrorWriter(), "fatal error: %v\n %#v\n", err, errors.Unwrap(err))
 		os.Exit(ec)
 	}
 }