cmd/geth, jsre: improve the js command

geth js stopped the JS runtime after running the first input file
and blocked for pending callbacks. This commit makes it process
all files and enables quitting with Ctrl-C regardless of callbacks.

Error reporting is also improved. If a script fails to load, the error
is printed and includes the backtrace. package jsre now ensures that
otto is aware of the filename, the backtrace will contain them.

Before:

$ geth js bad.js; echo "exit $?"
... log messages ...
exit 0

After:

$ geth js bad.js; echo "exit $?"
... log messages ...
Fatal: JavaScript Error: Invalid number of input parameters
    at web3.js:3109:20
    at web3.js:4917:15
    at web3.js:4960:5
    at web3.js:4984:23
    at checkWork (bad.js:11:9)
    at bad.js:19:1

exit 1

jsre/jsre.go

@@ -235,7 +235,14 @@ func (self *JSRE) Exec(file string) error {
 	if err != nil {
 		return err
 	}
-	self.Do(func(vm *otto.Otto) { _, err = vm.Run(code) })
+	var script *otto.Script
+	self.Do(func(vm *otto.Otto) {
+		script, err = vm.Compile(file, code)
+		if err != nil {
+			return
+		}
+		_, err = vm.Run(script)
+	})
 	return err
 }