give a detailed error message in case of indentation mismatches in multiline jsonnet blocks #331

Job	Run time
Build and test Python module	1m 25s
Build and test using Bazel	1m 14s
Build and test using CMake	1m 19s
Build and test using plain Makefile	1m 58s
Build Bazel example (8.*)	43s
Build Bazel example (7.*)	42s
Build Bazel example (9.*)	1m 29s
Check code formatting	0s
	8m 50s

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