Replace try_method! macro with an explicit predicate and inline matches

[demoray]

try_method! in src/snapshot.rs had three exit paths embedded in
one expression: succeed-and-return-from-caller, fail-fast-and-return-
from-caller, or evaluate to a Box<Error> for aggregation. Two of the
three reached into the enclosing function's control flow, so reading

let crash = try_method!(self.create_source(&Source::DevCrash));

looked like assignment but might never bind. That's the kind of
hidden control flow macros should avoid.

Extract the disk-usage special case into a named predicate on Error,
and rewrite the three call sites in Snapshot::create() as explicit
match arms. Behavior is unchanged: success returns early, a
disk-usage rejection surfaces immediately, and any other failure is
boxed and aggregated into AllSourcesFailed { crash, kcore, devmem }.
The macro is deleted.

Wins: every return is visible at its call site; the predicate has
a name and is testable in isolation; no macros hiding control flow.

Verified with:

• cargo fmt --check
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test --all-features