subscriber: fix clippy lints

[hawkw]

Motivation

Looks like Clippy is once again complaining about the
match_wild_err_arm lint, presumably as a result of the Rust 1.42
release. This lint triggers on the try_lock! macro that
tracing-subscriber uses to avoid double panics when a mutex is
poisoned. In this case, the lint is something of a false positive here,
since we do actually have two different Err(...) arms; the
differentiation between the two arms is not in the match pattern but in
a guard. See rust-lang/rust-clippy#3688 for
details on the lint.

Solution

I've refactored the code in question to use if/else, avoiding the
lint.

Signed-off-by: Eliza Weisman eliza@buoyant.io