log previous exception message from error writer

it's not always clear from an exception message what the underlying issue was, so
also print out the previous exception's message, if available.

src/API/Behavior/Internal/LogWriter/Formatter.php

@@ -12,11 +12,13 @@ public static function format($level, string $message, array $context): string
             ? $context['exception']
             : null;
         if ($exception) {
+            $previous = $exception->getPrevious() ? $exception->getPrevious()->getMessage() : '';
             $message = sprintf(
-                'OpenTelemetry: [%s] %s [exception] %s%s%s',
+                'OpenTelemetry: [%s] %s [exception] %s [previous] %s%s%s',
                 $level,
                 $message,
                 $exception->getMessage(),
+                $previous,
                 PHP_EOL,
                 $exception->getTraceAsString()
             );