Fix crash when writing a non-existent attribute

Yury Matveevrequested to merge
fix-785-write-nonexistent-attribute into develop

Writing an attribute that does not exist swallowed the DevFailed from the configuration lookup and converted the value against an uninitialised AttributeInfoEx, which segfaulted on macOS and raised a misleading "unsupported data_format" on Linux. The error is now propagated, so write_attribute reports API_AttrNotFound like write_attributes already did.

The attribute type dispatch fell back to assert(false), a no-op in release builds, so an unhandled data type silently skipped the conversion and left the caller with unset data; it now raises a TypeError naming the type. This reaches all dispatch sites, which all hold the GIL, and also fixes a hang when a caller-built AttributeInfoEx is passed to write_attribute.

Closes #785 (closed)

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