Show error message when failed to start preview command (junegunn#1810)

Fix junegunn#1637

Author: tonymet

.gitignore

@@ -8,3 +8,4 @@ doc/tags
 vendor
 gopath
 *.zwc
+fzf

README.md

@@ -480,7 +480,8 @@ See *KEY BINDINGS* section of the man page for details.
 ### Preview window
 
 When `--preview` option is set, fzf automatically starts an external process with
-the current line as the argument and shows the result in the split window.
+the current line as the argument and shows the result in the split window. Your
+`$SHELL` is used to execute the command with `$SHELL -c COMMAND`
 
 ```bash
 # {} is replaced to the single-quoted string of the focused line

src/terminal.go

@@ -1576,7 +1576,10 @@ func (t *Terminal) Loop() {
 					var out bytes.Buffer
 					cmd.Stdout = &out
 					cmd.Stderr = &out
-					cmd.Start()
+					err := cmd.Start()
+					if err != nil {
+						out.Write([]byte(err.Error()))
+					}
 					finishChan := make(chan bool, 1)
 					updateChan := make(chan bool)
 					go func() {