improve antlr import error message

[masahi]

Currently, the error message I get from the parser import failure is confusing.

If I try to use VM functionality, I get an error Couldn't find ANTLR parser. Try building with USE_ANTLR=ON. But even if I rebuild TVM with USE_ANTLR=ON, I still get the same error because the generated parser try to import the antlr4 python package that I haven't installed, but the error still says Try building with USE_ANTLR=ON.

This is confusing, I think

try:
    from antlr4 import InputStream, CommonTokenStream
    from antlr4.error.ErrorListener import ErrorListener
except ImportError:
    raise Exception("Couldn't find ANTLR runtime." +
                    "Try running `pip{version} install antlr4-python{version}-runtime`."
                    .format(version=PYTHON_VERSION))

should come before

try:
    from .grammar.py3.RelayVisitor import RelayVisitor
    from .grammar.py3.RelayParser import RelayParser
    from .grammar.py3.RelayLexer import RelayLexer
except ImportError:
    raise Exception("Couldn't find ANTLR parser. Try building with USE_ANTLR=ON.")

because the latter depends on the antlr4 python package.

please review @wweic @zhiics @icemelon9

[icemelon]

lgtm

[icemelon]

Thanks @masahi