Emit more accurate error message (#760)

* Emit more accurate error message

Until issue #645 is resolved, this is a  measure to not mislead the user.

* Fix test

Author: goretkin

src/PyCall.jl

@@ -495,7 +495,7 @@ function pyimport(name::AbstractString)
             if pyisinstance(e.val, @pyglobalobjptr(:PyExc_ImportError))
                 # Expand message to help with common user confusions.
                 msg = """
-The Python package $name could not be found by pyimport. Usually this means
+The Python package $name could not be imported by pyimport. Usually this means
 that you did not install $name in the Python version being used by PyCall.
 
 """

test/runtests.jl

@@ -353,7 +353,7 @@ const PyInt = pyversion < v"3" ? Int : Clonglong
         ex
     end
     @test ex isa PyCall.PyError
-    @test occursin("could not be found by pyimport", ex.msg)
+    @test occursin("could not be imported by pyimport", ex.msg)
     # Make sure we are testing ModuleNotFoundError here:
     if PyCall.pyversion >= v"3.6"
         @test pyisinstance(ex.val, pybuiltin("ModuleNotFoundError"))