bpmodify: respect exitCode

main() method calls os.Exit(exitCode) upon return, respecting any
exitCode set by the report() method.

Test: bpmodify -w; echo "$?"
Test: bpmodify Android.bp; echo "$?"
Test: # echo command should output "2"
Change-Id: Iaf056301eaba3f249b256ecf0f0d87f8a4df1c58