Use named fields on UnableToCreateSnapshotFromSource; escape Raw paths in Display

[demoray]

Two surgical changes in src/snapshot.rs.

1. Named fields on UnableToCreateSnapshotFromSource

// before
UnableToCreateSnapshotFromSource(#[source] Box<Error>, Source)

// after
UnableToCreateSnapshotFromSource {
    src: Source,
    #[source]
    source: Box<Error>,
}

It was the only positional-tuple variant in this error enum —
Error::Io, Error::WriteBlock, Error::DiskUsageEstimateExceeded,
and Error::Other all use named fields. The construction site in
create_source is now self-documenting:

.map_err(|e| Error::UnableToCreateSnapshotFromSource {
    src: src.clone(),
    source: Box::new(e),
})

2. Escape Source::Raw paths in Display

// before
Self::Raw(ref path) => write!(f, "{}", path.display()),

// after
Self::Raw(ref path) => write!(f, "{path:?}"),

path.display() is lossy-but-prints-anything — control characters,
ANSI escapes, embedded newlines all flow verbatim into error messages
that may be logged or rendered in CI annotations. PathBuf's Debug
quotes and escapes, which is the right default inside a structured
error.

Scope and compatibility

• One file, +12/−5.
• Internal callers updated: the one pattern match (inside the
  try_method! macro; Replace try_method! macro with an explicit predicate and inline matches #801 deletes the macro entirely but this PR
  stands alone either way) and the one construction site.
• Error is pub, so the variant rename is technically visible to
  any external match arm. The only documented downstream is this
  crate's own custom Debug impl, which goes through Display.

Not included

The larger restructure I described — splitting Source into a
KernelSource enum (the three well-known kernel devices, fully
ValueEnum-exposed) plus a separate Snapshot::raw_source(path)
builder method — is a public-API change worth raising as an issue
first. Source::Raw is currently #[value(skip)], so it's library-
only and doesn't participate in the auto-detect fallback; that split
would tighten the type but break any external library consumer.

Verified with:

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