enable gzip compression (for http reponse)
handle error in metafont generation and display error message
timeout mf2outline after 5 seconds

in some cases, the mf2outline script ran longer than the webserver timeout
allowed, which resulted in the webserver returning a response but the python script
going on without being killed.
and furthermore, waiting more than 5 seconds is not reasonable for the end user.
add better errors gem
require libs/gems only for required environments

* sinatra reloader: dev
* time: prod
remove Rack::ShowExceptions (not needed i guess)
handle error for export also

see 1b2159f for the initial implementation on handling the generation error.
this change now handles also the case for font export.

additionally, we first generate the export by ajax and only after redirect to
the download. by adding caching the 2nd call to the same export should not
be a huge overhead.
remove output file before each otf generation

if we leave it, the error check after the generation can be successful even
though the current pass did not succeed and did not generate an otf.