blog: show a helpful error for invalid template directory
blog: show a helpful error for invalid template directory

Currently, if the blog binary is executed outside the root directory
without any additional flags, it bails out with the following error -
"open template/root.tmpl: no such file or directory"

This CL prints out a more user friendly error which allows the user to
learn that there is a flag with which the template directory can be set.

Fixes golang/go#22622

Change-Id: I726e7c28f5e5036146769ca01516368f45a6262c
Reviewed-on: https://go-review.googlesource.com/86855
Run-TryBot: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools