gracefully handle out of disk space failures

Describe the bug grin fails to recover gracefully from a crash resulting from running out of disk space.

To Reproduce Steps to reproduce the behavior:

1. Run grin
2. Run out of disk space; grin panics
3. Increase disk space and restart grin

Relevant Information

20200817 16:51:27.347 INFO grin - Using configuration file at /home/jameson/.grin/main/grin-server.toml
20200817 16:51:27.347 INFO grin - This is Grin version 4.0.2 (git v4.0.2), built for x86_64-unknown-linux-gnu by rustc 1.45.2 (d3fb005a3 2020-07-31).
20200817 16:51:27.347 DEBUG grin - Built with profile "release", features "".
20200817 16:51:27.347 INFO grin - Chain: Mainnet
20200817 16:51:27.347 INFO grin - Feature: NRD kernel enabled: false
20200817 16:51:27.347 WARN grin::cmd::server - Starting GRIN in UI mode...
20200817 16:51:27.354 INFO grin_servers::grin::server - Starting server, genesis block: 40adad0aec27
20200817 16:51:27.358 DEBUG grin_store::lmdb - DB Mapsize for /home/jameson/.grin/main/chain_data/lmdb is 549755813888
20200817 16:51:27.431 DEBUG grin_store::leaf_set - bitmap 162820 pos (315706 bytes)
20200817 16:51:29.779 DEBUG grin_store::prune_list - bitmap 478437 pos (718704 bytes), pruned_cache 6843301 pos (772299 bytes), shift_cache 478437, leaf_shift_cache 478437
20200817 16:51:29.920 DEBUG grin_store::leaf_set - bitmap 162820 pos (315706 bytes)
20200817 16:51:32.365 DEBUG grin_store::prune_list - bitmap 478437 pos (718704 bytes), pruned_cache 6843301 pos (772299 bytes), shift_cache 478437, leaf_shift_cache 478437
20200817 16:51:32.407 DEBUG grin_chain::txhashset::bitmap_accumulator - applied 3777 chunks from idx 0 to idx 3776 (41ms)
20200817 16:51:34.074 DEBUG grin_chain::txhashset::txhashset - attempting to open kernel PMMR using ProtocolVersion(2) - FAIL (verify failed)
20200817 16:51:34.117 DEBUG grin_chain::txhashset::txhashset - attempting to open kernel PMMR using ProtocolVersion(1) - SUCCESS
20200817 16:51:34.325 ERROR grin_util::logger - 
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Chain(Error { inner: 

Other Error: failed to find head hash })': src/bin/cmd/server.rs:48   0: grin_util::logger::send_panic_to_log::{{closure}}
   1: std::panicking::rust_panic_with_hook
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panicking.rs:490
   2: rust_begin_unwind
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panicking.rs:388
   3: core::panicking::panic_fmt
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libcore/panicking.rs:101
   4: core::option::expect_none_failed
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libcore/option.rs:1272
   5: grin::cmd::server::start_server_tui
   6: grin::cmd::server::server_command
   7: grin::real_main
   8: grin::main
   9: std::rt::lang_start::{{closure}}
  10: std::rt::lang_start_internal::{{closure}}
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/rt.rs:52
      std::panicking::try::do_call
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panicking.rs:297
      std::panicking::try
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panicking.rs:274
      std::panic::catch_unwind
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panic.rs:394
      std::rt::lang_start_internal
             at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/rt.rs:51
  11: main
  12: __libc_start_main
  13: _start

Desktop (please complete the following information):

• OS: Ubuntu 18.04

I ran grin --clean and it appears to have wiped all 6 GB of chain data; the node is now resyncing from genesis.