MgReader.GetPropertyIndex() on an invalid property name should return -1

[jng]

Currently, calling MgReader.GetPropertyIndex() on an invalid property name will throw a MgObjectNotFoundException.

In such cases, returning -1 is sufficient to indicate an invalid property.

[jng]

In 9369:

Don't throw on bad property name in GetPropertyIndex, just return -1. This is the expected contract that other reader types already fulfil. Why should MgFeatureReader be an exception?

Fixes #2480

[jng]

In 9370:

Merged revision(s) 9369 from trunk/MgDev:
Don't throw on bad property name in GetPropertyIndex, just return -1. This is the expected contract that other reader types already fulfil. Why should MgFeatureReader be an exception?

Fixes #2480
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