Log to dropbox if we fail to create an ANR stack trace file.

Some ANRs have no stack traces uploaded through dropbox. This appears to be because the file holding them was not created

So in the case where we can't create the file, add an exception message to the string part of the dropbox upload.

Change-Id: Ic06ac9c25974633905b1a0bfcfcef8ed833795e8
Bug: 148848039
Test: Manually threw exception in createAnrDumpFile() and triggered ANR.