init: fix error messages when an invalid section header is found

If a section header such as 'on' or 'service' has an error with it,
the rest of the commands / options are currently reported as errors
since there is no valid parser object to parse them.  For example,

service !@#$%%^&*() /system/bin/false
    user root
    group blah
    blah blah

Generates:

init: /system/etc/init/bad.rc: 2: invalid service name '!@#$%%^&*()'
init: /system/etc/init/bad.rc: 3: Invalid section keyword found
init: /system/etc/init/bad.rc: 4: Invalid section keyword found
init: /system/etc/init/bad.rc: 5: Invalid section keyword found

This change suppresses the extraneous 'Invalid section keyword found'
messages.

Test: faulty error messages are suppressed.
Change-Id: Ieeb2d5b8b7eea33e191a88ce5a0d41701686943f