Change the way the input framework deals with values it cannot convert

into BroVals (especially enums)

Not we do not force an internal error anymore. Instead, we raise an
normal error and set an error flag that signals to the top-level
functions that the value could not be converted and should not be
propagated to the Bro core. This sadly makes the already messy code even
more messy - but since errors can happen in deeply nested data
structures, the alternative (catching the error at every possible
location and then trying to clean up there instead of recursively
deleting the data that cannot be used later) is much worse.

Addresses BIT-1199

testing/btest/Baseline/scripts.base.frameworks.input.missing-enum/bro..stdout

@@ -0,0 +1,4 @@
+Table:
+{
+
+}