fix: log command output when conversion finished not found

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

lib/Service/ConversionService.php

@@ -61,7 +61,7 @@ public function convert(string $filePath): string {
 		}
 
 		if (str_contains($stdout, 'Conversion finished') === false) {
-			throw new ConversionException('Conversion did not finish');
+			throw new ConversionException('The command output does not contain the phrase "Conversion finished". Output: ' . $stdout);
 		}
 
 		$status = proc_close($process);