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/db.rs

@@ -1213,7 +1213,8 @@ impl LockedDatabase {
                 open.id = o.id;
                 open.uuid.extend_from_slice(&o.uuid.as_bytes()[..]);
             }
-            let d = dir::SampleFileDir::open(&dir.path, &meta)?;
+            let d = dir::SampleFileDir::open(&dir.path, &meta)
+                .map_err(|e| e.context(format!("Failed to open dir {}", dir.path)))?;
             if self.open.is_none() {  // read-only mode; it's already fully opened.
                 dir.dir = Some(d);
             } else {  // read-write mode; there are more steps to do.