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/base/clock.rs

@@ -64,7 +64,7 @@ where C: Clocks, E: Into<Error> {
             Err(e) => e.into(),
         };
         let sleep_time = Duration::seconds(1);
-        warn!("sleeping for {:?} after error: {:?}", sleep_time, e);
+        warn!("sleeping for {:?} after error: {}", sleep_time, crate::error::prettify_failure(&e));
         clocks.sleep(sleep_time);
     }
 }

server/base/error.rs

@@ -29,7 +29,22 @@
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 use failure::{Backtrace, Context, Fail};
-use std::fmt;
+use std::fmt::{self, Write};
+
+/// Returns a pretty-and-informative version of `e`.
+pub fn prettify_failure(e: &failure::Error) -> String {
+    let mut msg = e.to_string();
+    for cause in e.iter_causes() {
+        write!(&mut msg, "caused by: {}", cause).unwrap();
+    }
+    if e.backtrace().is_empty() {
+        write!(&mut msg, "\n(set environment variable RUST_BACKTRACE=1 to see backtraces)")
+            .unwrap();
+    } else {
+        write!(&mut msg, "\nBacktrace:\n{}", e.backtrace()).unwrap();
+    }
+    msg
+}
 
 #[derive(Debug)]
 pub struct Error {

server/base/lib.rs

@@ -33,4 +33,4 @@ pub mod time;
 mod error;
 pub mod strutil;
 
-pub use crate::error::{Error, ErrorKind, ResultExt};
+pub use crate::error::{Error, ErrorKind, ResultExt, prettify_failure};