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.

guide/troubleshooting.md

@@ -15,7 +15,7 @@ While Moonfire NVR is running, logs will be written to stderr.
      likely want to set `MOONFIRE_FORMAT=google-systemd` to format logs as
      expected by systemd.
 
-Logging options are controlled by environmental variables:
+Logging options are controlled by environment variables:
 
    * `MOONFIRE_LOG` controls the log level. Its format is similar to the
      `RUST_LOG` variable used by the
@@ -27,6 +27,7 @@ Logging options are controlled by environmental variables:
      accepted are `google` (the default, like the Google
      [glog](https://github.com/google/glog) package) and `google-systemd` (a
      variation for better systemd compatibility).
+   * Errors include a backtrace if `RUST_BACKTRACE=1` is set.
 
 If you use Docker, set these via Docker's `--env` argument.