glave-generate: Replace asserts with proper error detection and error logging.

glave-generate: Replace asserts with proper error detection and error logging.

* On Linux debug builds, the asserts cause the debugger to abort, but the code wasn't handling the error case safely anyway, so even a release build would likely crash.
* These situations were being hit if trying to load a trace file from an earlier header version, but are likely to be conditions we'll see when new developers start to use the API.
* With these changes, the error is properly reported and error return values propogated so that the UI detects the invalid file and reports an error to the user.
* The new glv_LogError(..) messages get printed to the console, but are not yet passed to the UI. This will be done as part of a larger change sometime in the future.