Display the broken julia_cmd on error so the user can identify the problem

[NHDaly]

It looks like PackageCompiler isn't compatible with Julia v1.1.0: the command-line args in Base.julia_cmd() have changed.

This PR is a tiny one that just helps the user see what's wrong in the output:

ERROR: LoadError: Unexpected format of "Base.julia_cmd()", you may be using an incompatible version of Julia:
`/Users/nathan.daly/src/julia_release_native/usr/bin/julia -Cnative -J/Users/nathan.daly/src/julia_release_native/usr/lib/julia/sys.dylib --check-bounds=yes -g1`

I'll follow up with an actual fix to the above v1.1 problem! :)

[NHDaly]

Actually, i don't really see why we have this check at all... We may as well give it a shot, no? I don't see why the user shouldn't be allowed to start julia with custom flags and have those carry-through when compiling (e.g. like --math-mode=fast or something)

[NHDaly]

Ah, jk i get it, it's cause we want to assert that the structure matches what we expect so we can deal with it in the following line. :)

K nvm, I think this PR should be merged as is, and i'll work on a fix for v1.1 in a follow-up! :)

[NHDaly]

(The build failures are for nightly, which of course we know is broken. But the good news is now at least the errors include the context:

ERROR: LoadError: Unexpected format of "Base.julia_cmd()", you may be using an incompatible version of Julia:
`/home/travis/julia/bin/julia -Cnative -J/home/travis/julia/lib/julia/sys.so --check-bounds=yes -g1 --code-coverage=user`

https://travis-ci.org/JuliaLang/PackageCompiler.jl/jobs/483001973)

[codecov-io]

Codecov Report

Merging #166 into master will increase coverage by 0.92%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   61.72%   62.65%   +0.92%     
==========================================
  Files           6        6              
  Lines         324      324              
==========================================
+ Hits          200      203       +3     
+ Misses        124      121       -3

Impacted Files	Coverage Δ
src/static_julia.jl	64.7% <0%> (ø)	⬆️
src/PackageCompiler.jl	45% <0%> (+1.66%)	⬆️
src/system_image.jl	70.96% <0%> (+6.45%)	⬆️

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[lucatrv]

Yes I confirm that I put that check to assert that the structure matches what we expect.