panic on prune

[bjo81]

Hi,
thanks for #235 , got a new crash now:

name = "rustic-rs"
operating_system = "Arch Linux [64-bit]"
crate_version = "0.7.0-dev"
explanation = """
Panic occurred in file '/data/build/.cargo/git/checkouts/rustic_core-fa42f42e094d79df/1cecce8/crates/core/src/blob/tree.rs' at line 690
"""
cause = 'called `Result::unwrap()` on an `Err` value: "SendError(..)"'
method = "Panic"
backtrace = """

   0: 0x61ebd4d5e1da - <unresolved>
   1: 0x61ebd4d5b7c7 - <unresolved>
   2: 0x61ebd4ffa9cb - <unresolved>
   3: 0x7335ac51839d - <unresolved>
   4: 0x7335ac59d49c - <unresolved>
   5:        0x0 - <unresolved>"""

[zhuxiujia]

code have many .unwrap(); on 45 matches on 15 files.
I suggest the author should handle these possible exceptions，may make it more stable