docopt.cpp: wrong arguments throw runtime_error

Even though that thing is documented to throw an std::invalid_argument,
it actually throws std::runtime_error, and has been doing that for
years. Yay, documentation.

Change-Id: If93dff37e80e5f82db0fd556bc3e2d198216b90a
diff --git a/src/utils/log.cpp b/src/utils/log.cpp
index f6fe716..9f39fb4 100644
--- a/src/utils/log.cpp
+++ b/src/utils/log.cpp
@@ -13,7 +13,7 @@
     long x;
     try {
         x = option.asLong();
-    } catch (std::invalid_argument&) {
+    } catch (std::runtime_error&) {
         throw std::runtime_error(name + " log level: expecting integer");
     }
     static_assert(spdlog::level::trace < spdlog::level::off, "spdlog::level levels have changed");