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>