Add backtraces to errors (#1498)

Error handling has been reworked to always go through the new `error_template`,
`error_json` and `error_atom` macros.
They all accept a status code followed by a string message or an exception
object. `error_json` accepts a hash with additional fields as third argument.

If the second argument is an exception a backtrace will be printed, if it is a
string only the string is printed. Since up till now only the exception message
was printed a new `InfoException` class was added for situations where no
backtrace is intended but a string cannot be used.

`error_template` with a string message automatically localizes the message.
Missing error translations have been collected in https://github.com/iv-org/invidious/issues/1497
`error_json` with a string message does not localize the message. This is the
same as previous behavior. If translations are desired for `error_json` they
can be added easily but those error messages have not been collected yet.

Uncaught exceptions previously only printed a generic message ("Looks like
you've found a bug in Invidious. [...]"). They still print that message
but now also include a backtrace.

src/invidious/mixes.cr

@@ -30,7 +30,7 @@ def fetch_mix(rdid, video_id, cookies = nil, locale = nil)
   initial_data = extract_initial_data(response.body)
 
   if !initial_data["contents"]["twoColumnWatchNextResults"]["playlist"]?
-    raise translate(locale, "Could not create mix.")
+    raise InfoException.new("Could not create mix.")
   end
 
   playlist = initial_data["contents"]["twoColumnWatchNextResults"]["playlist"]["playlist"]