Provide better error message when IJulia is installed somewhere other than the global environment

[LilithHafner]

Currently, when one installs IJulia in a temporary environment (e.g. to test out the package) the kernel fails to connect, with no obvious error message. This led me to raise issue #1126. Ideally, in this case there would be an error message or warning of some sort indicating what the problem is.

[LilithHafner]

Even better would be to not require that IJulia is installed in the global environment.

[JamesWrigley]

FWIW, I like to install it in a @ijulia environment and install the kernel with installkernel("Julia", "--project=@ijulia"). Then within each notebook I ] activate /path/to/notebook/environment at the beginning.