Remove is_method_error method

Remove the usage of is_method_error()[1], and add try-catch to handle
D-Bus exceptions around mapper call.

[1]: https://github.com/openbmc/sdbusplus/commit/079fb85a398d90800935e3985bb1266a7530a26e#diff-945669e8bd9cab4ecc83a574a732921281b2c79eb8bba65efff11736ad18f92bR237-R240

Change-Id: I46c8b7adf460acd15d22e627536733385a58715f
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/fault-monitor/fru-fault-monitor.cpp b/fault-monitor/fru-fault-monitor.cpp
index 7b323b2..7ce8cca 100644
--- a/fault-monitor/fru-fault-monitor.cpp
+++ b/fault-monitor/fru-fault-monitor.cpp
@@ -218,17 +218,11 @@
             "org.freedesktop.DBus.Properties", "Get");
         method.append("xyz.openbmc_project.Association.Definitions");
         method.append("Associations");
-        auto reply = bus.call(method);
-        if (reply.is_method_error())
-        {
-            // do not stop, continue with next elog
-            lg2::error("Error in getting associations");
-            continue;
-        }
 
         std::variant<AssociationList> assoc;
         try
         {
+            auto reply = bus.call(method);
             reply.read(assoc);
         }
         catch (const sdbusplus::exception_t& e)