Better error reporting on stpl

[mmacedoeu]

Got an error on Template compiling:

error: Failed to compile template.
Caused by: Rust Syntax Error: unexpected token

file: templates/mosaic_gstreamer.stpl

  --> src/cli.rs:10:10
   |
10 | #[derive(TemplateOnce)]
   |          ^^^^^^^^^^^^
   |
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

The message is too generic, don't know what and where this unexpected token appears

[Kogia-sima]

Needs dtolnay/proc-macro2#178 to be resolved