Fix potential miscellaneous debuggerd issues.

Check for the log opening failing.

Add the ability to put error messages in the log and tombstone so
that it's clear if the log reading failed in some way.

Adjust test so that if there is a log or if no log exists, the test
will still pass.

Print an <unknown> if the command line is unreadable instead of nothing.

Test: Ran unit tests.
Test: Induced error and verified error message is save in tombstone.
Change-Id: I2fce8078573b40b9fed3cd453235f3824cadb5e3