cxxll::dwarf::error: Use accessors
cxxll::dwarf::error::data(): New optional member
cxxll::elf_image::dwarf_cu_range: Check compilation unit tag
SQL schema: New column symboldb.dwarf_error.data
database::add_dwarf(): Load column symboldb.dwarf_error.data
cxxll::elf_image::tag(): New member function
SQL schema: Add column symboldb.dwarf_compilation_unit.tag
Integration test: Move comment
Integration test: Enhance RPM list testing
cxxll::pg_response(): Add an overload which accepts an rvalue reference

Technically, the argument should be a const pgresult_handle & instead
of a non-const reference, but this is difficult to express with the
current pgresult_handle implementation.
rpm_load test case: Automatically adjust to new RPMs

The RPM list is now covered by the integration test.
Adjust integration test to new DWARF columns
DWARF integration test: Use LEFT OUTER JOIN to cover NULL producer columns
Additional test RPMs: DWARF errors
cxxll::elf_image::dwarf_cu_range: Suppress some errors in partial CUs
SQL schema: Add column symboldb.dwarf_compilatuon_unit.die_offset

Adjust database loading code accordingly.
Integration test: Cover column symboldb.dwarf_compilation_unit.die_offset
cxxll::elf_image::dwarf_cu_range::cu_offset(): New member function
Add compilation unit offsets to DWARF tables

This also adds the cu_offset member to cxxll::dwarf::error,
and adjusts its constructors.