return error if feed is not found during sync

[ayrat555]

fixes

Feb 28 16:00:40 vps695005 bash[9137]: thread 'sync-pool' panicked at 'called `Option::unwrap()` on a `None` value', src/sync/feed_sync_job.rs:35:20
Feb 28 16:00:40 vps695005 bash[9137]: stack backtrace:
Feb 28 16:00:40 vps695005 bash[9137]:    0: backtrace::backtrace::libunwind::trace
Feb 28 16:00:40 vps695005 bash[9137]:              at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
Feb 28 16:00:40 vps695005 bash[9137]:    1: backtrace::backtrace::trace_unsynchronized
Feb 28 16:00:40 vps695005 bash[9137]:              at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
Feb 28 16:00:40 vps695005 bash[9137]:    2: std::sys_common::backtrace::_print_fmt
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/sys_common/backtrace.rs:78
Feb 28 16:00:40 vps695005 bash[9137]:    3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/sys_common/backtrace.rs:59
Feb 28 16:00:40 vps695005 bash[9137]:    4: core::fmt::write
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libcore/fmt/mod.rs:1069
Feb 28 16:00:40 vps695005 bash[9137]:    5: std::io::Write::write_fmt
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/io/mod.rs:1537
Feb 28 16:00:40 vps695005 bash[9137]:    6: std::sys_common::backtrace::_print
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/sys_common/backtrace.rs:62
Feb 28 16:00:40 vps695005 bash[9137]:    7: std::sys_common::backtrace::print
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/sys_common/backtrace.rs:49
Feb 28 16:00:40 vps695005 bash[9137]:    8: std::panicking::default_hook::{{closure}}
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/panicking.rs:198
Feb 28 16:00:40 vps695005 bash[9137]:    9: std::panicking::default_hook
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/panicking.rs:218
Feb 28 16:00:40 vps695005 bash[9137]:   10: std::panicking::rust_panic_with_hook
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/panicking.rs:477
Feb 28 16:00:40 vps695005 bash[9137]:   11: rust_begin_unwind
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libstd/panicking.rs:385
Feb 28 16:00:40 vps695005 bash[9137]:   12: core::panicking::panic_fmt
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libcore/panicking.rs:89
Feb 28 16:00:40 vps695005 bash[9137]:   13: core::panicking::panic
Feb 28 16:00:40 vps695005 bash[9137]:              at src/libcore/panicking.rs:52
Feb 28 16:00:40 vps695005 bash[9137]:   14: el_monitorro::sync::feed_sync_job::FeedSyncJob::execute