fix: cleanup for older libsystemd versions (#454)

Fix inconsistencies in documentation and CMake files regarding minimum required libsystemd version (which is v238 for sdbus-c++ v2.0). Also fix compiler error bug in sdbus-c++ code when compiled against older libsystemd versions. Plus some extra related cleanups.

docs/using-sdbus-c++.md

@@ -113,7 +113,7 @@ $ ninja libsystemd.so.0.26.0  # or another version number depending which system
 
 sdbus-c++ provides `SDBUSCPP_BUILD_LIBSYSTEMD` configuration option. When turned on, sdbus-c++ will automatically download and build libsystemd as a static library and make it an opaque part of sdbus-c++ shared library for you. This is the most convenient and effective approach to build, distribute and use sdbus-c++ as a self-contained, systemd-independent library in non-systemd environments. Just make sure your build machine has all dependencies needed by libsystemd build process. That includes, among others, `meson`, `ninja`, `git`, `gperf`, and -- primarily -- libraries and library headers for `libmount`, `libcap` and `librt` (part of glibc). Also, when distributing, make sure these dependency libraries are installed on the production machine.
 
-You may additionally set the `SDBUSCPP_LIBSYSTEMD_VERSION` configuration flag to fine-tune the version of systemd to be taken in. (The default value is 242).
+You may additionally set the `SDBUSCPP_LIBSYSTEMD_VERSION` configuration flag to fine-tune the version of systemd to be taken in. (The default value is 242, the minimum value is 239).
 
 Distributing sdbus-c++
 ----------------------
@@ -854,7 +854,7 @@ protected:
     {
         // Return error if there are no numbers in the collection
         if (numbers.empty())
-            throw sdbus::Error("org.sdbuscpp.Concatenator.Error", "No numbers provided");
+            throw sdbus::Error(sdbus::Error::Name{"org.sdbuscpp.Concatenator.Error"}, "No numbers provided");
 
         // Concatenate the numbers
         std::string result;