Use the PDF's existence to avoid sending duplicate emails

If the PDF is already on disk, it's a good sign that we've already sent
the email, so skip in that case.

To make the signal a bit more reliable, make writing the PDF to disk
the last action taken, in case sending the email fails.

Cargo.toml

@@ -16,6 +16,5 @@ ureq = { version = "*", features = ["cookies"] }
 codegen-units = 1
 lto = true
 opt-level = "z"
-panic = "abort"
 strip = true