[Vertex AI] Renamed `RPCError` to `BackendError` #734

Job	Run time
pod-lib-lint (ios, macos-14)	7m 4s
spm-package-resolved	1m 11s
testapp-integration (iOS, macos-15)	4m 43s
spm-unit (macos-13, Xcode_15.2, iOS)	5m 43s
sample (macos-13, Xcode_15.2)	4m 54s
spm-unit (macos-14, Xcode_15.4, iOS)	8m 32s
sample (macos-14, Xcode_15.4)	3m 8s
spm-unit (macos-15, Xcode_16, iOS)	2m 1s
sample (macos-15, Xcode_16)	2m 38s
spm-unit (macos-15, Xcode_16, tvOS)	1m 52s
spm-unit (macos-15, Xcode_16, macOS)	1m 43s
spm-unit (macos-15, Xcode_16, watchOS)	1m 59s
spm-unit (macos-15, Xcode_16, catalyst)	1m 45s
spm-unit (macos-15, Xcode_16, visionOS)	2m 40s
	49m 53s

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