Since we allow arbitrary code execution in the generation of the fields
for document indexing, there are cases where ::meta/doc values call
search when the search index is empty.

There are an infinite number of other cases in which userland code could
throw an exception during indexing.

Rather than have the entire search index be empty if one document throws
an exception, these changes print to stderr and record in an atom which
imetas failed to index. The message printed to stderr suggests a remedy
for indexing the specific imetas that failed.