browser-plugin: Fix some function error returns

Returning values when they weren’t supposed to.
plugins: Attempt to silence some warnings unavoidably produced by Vala

This won’t work if the -Wno-* flags are later overridden by custom CFLAGS,
but should work to silence spurious warnings about generated C code
otherwise.
chapters: Fix a documentation comment’s return line
grilo: Fix a function error return

It wasn’t returning a value in a non-void function.
rotation: Use explicit .begin()/.end for async Vala calls

This removes a compilation warning and makes the code a little clearer.
zeitgeist-dp: Fix some Vala warnings

Use explicit .begin()/.end() for an async call, and handle a GError
which wasn’t previously handled.
screenshot: Attempt to silence warnings about printf() formatting

A format string is passed through a couple of functions in the
screenshot plugin, hence gcc warns about using a variable format
string in a printf() call. Attempt to silence this by annotating
the functions with G_GNUC_FORMAT() and also disabling that warning
for the plugin.
core: Fix return type line in a documentation comment