Fix "ex" variable collision in invidious.cr

The exception handling for database connections results in an
`ex` variable which Ameba sees as overshadowing the `ex` used by the
`ex` block arg used to define the HTTP status code 500 handler below.

Although this is a non-issue since the db connection exception handling
will cause Invidious to exit, Ameba's nature as a static checker means
that it isn't aware of this.

The simplest fix without a dirty ameba ignore comment is to rename `ex`
within the Kemal handler block below, since `ex` within a begin rescue
block is a Crystal convention that will also cause Ameba to raise when
not adhered to.

src/invidious.cr

@@ -62,8 +62,8 @@ HMAC_KEY = CONFIG.hmac_key
 
 PG_DB = begin
   DB.open CONFIG.database_url
-rescue exc
-  puts "Failed to connect to PostgreSQL database: #{exc.cause.try &.message}"
+rescue ex
+  puts "Failed to connect to PostgreSQL database: #{ex.cause.try &.message}"
   puts "Check your 'config.yml' database settings or PostgreSQL settings."
   exit(1)
 end
@@ -227,8 +227,8 @@ error 404 do |env|
   Invidious::Routes::ErrorRoutes.error_404(env)
 end
 
-error 500 do |env, ex|
-  error_template(500, ex)
+error 500 do |env, exception|
+  error_template(500, exception)
 end
 
 static_headers do |env|