Work around invalid code_file strings in Minidump modules

[loewenheim]

Currently we reject minidumps in which some modules have garbled code_files outright. It would be better to stackwalk the rest of the minidump and just skip the offending modules. To this end, we would need to:

1. Log an error when we encounter an invalid code_file (this already happens)
2. Display some placeholder string like <invalid> or <corrupted> in the UI
3. Replace the useless name with None in derived types that perform lookups based on the code_file