Print a friendly instruction upon ImportError

[tkf]

fixes #4

[codecov-io]

Codecov Report

Merging #5 into master will increase coverage by 15.04%.
The diff coverage is 72.61%.

@@             Coverage Diff             @@
##           master       #5       +/-   ##
===========================================
+ Coverage   57.89%   72.94%   +15.04%     
===========================================
  Files           1        4        +3     
  Lines          19       85       +66     
===========================================
+ Hits           11       62       +51     
- Misses          8       23       +15

Impacted Files	Coverage Δ
src/IPython.jl	100% <ø> (+42.1%)	⬆️
src/julia_repl.jl	53.33% <53.33%> (ø)
src/core.jl	75% <75%> (ø)
src/convenience.jl	76.92% <76.92%> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 40

• 61 of 84 (72.62%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+15.05%) to 72.941%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/core.jl	3	4	75.0%
src/julia_repl.jl	8	15	53.33%
src/convenience.jl	50	65	76.92%

Totals
Change from base Build 14:	15.05%
Covered Lines:	62
Relevant Lines:	85

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💛 - Coveralls