overhaul error messages

Inspired by the poor error message here:
https://github.com/scottlamb/moonfire-nvr/issues/107#issuecomment-777587727

*   print the friendlier Display version of the error rather than Debug.
    Eg, "EROFS: Read-only filesystem" rather than "Sys(EROFS)". Do this
    everywhere: on command exit, on syncer retries, and on stream
    retries.
*   print the most immediate problem and additional lines for each
    cause.
*   print the backtrace or an advertisement for RUST_BACKTRACE=1 if it's
    unavailable.
*   also mention RUST_BACKTRACE=1 in the troubleshooting guide.
*   add context in various places, including pathnames. There are surely
    many places more it'd be helpful, but this is a start.
*   allow subcommands to return failure without an Error.
    In particular, "moonfire-nvr check" does its own error printing
    because it wants to print all the errors it finds. Printing "see
    earlier errors" with a meaningless stack trace seems like it'd just
    confuse. But I also want to get rid of the misleading "Success" at
    the end and 0 return to the OS.

server/db/upgrade/v4_to_v5.rs

@@ -146,7 +146,8 @@ pub fn run(_args: &super::Args, tx: &rusqlite::Transaction) -> Result<(), Error>
         }
 
         let dir = dir::Fd::open(path, false)?;
-        dir.lock(FlockArg::LockExclusiveNonblock)?;
+        dir.lock(FlockArg::LockExclusiveNonblock)
+            .map_err(|e| e.context(format!("unable to lock dir {}", path)))?;
 
         let mut need_sync = maybe_upgrade_meta(&dir, &db_meta)?;
         if maybe_cleanup_garbage_uuids(&dir)? {