fix: Enum variant named Error causes ambiguous item

[caspermeijn]

The generated code uses Self::Error to refer to ::prost::UnknownEnumValue, but the term Error is not unique when an enum variant is called Error. Use the full type name to resolve the ambiguity.

The compiler reported:

error: ambiguous associated item
   --> /home/ircdev/src/prost/target/debug/build/tests-no-std-e27db7197924752f/out/enum_keyword_variant.rs:2:68
    |
2   | #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, ::prost::Enumeration)]
    |                                                                    ^^^^^^^^^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #57644 <https://github.com/rust-lang/rust/issues/57644>
note: `Error` could refer to the variant defined here
   --> /home/ircdev/src/prost/target/debug/build/tests-no-std-e27db7197924752f/out/enum_keyword_variant.rs:10:5
    |
10  |     Error = 4,
    |     ^^^^^
note: `Error` could also refer to the associated type defined here
   --> /home/ircdev/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:682:5
    |
682 |     type Error;
    |     ^^^^^^^^^^
    = note: `#[deny(ambiguous_associated_items)]` on by default
    = note: this error originates in the derive macro `::prost::Enumeration` (in Nightly builds, run with -Z macro-backtrace for more info)

Fixes #1097

[caspermeijn]

CIFuzz is failing due to a problem not related to this change: google/oss-fuzz#12168