more informative error message when output container does not exist

Author: samtkaplan

src/AzStorage.jl

@@ -749,6 +749,10 @@ cp(AzContainer("mycontainer";storageaccount="mystorageaccount"), "remoteblob_in.
 ```
 """
 function Base.cp(in::AbstractString, outc::AzContainer, outb::AbstractString; buffersize=2_000_000_000, show_progress=false)
+    if !isdir(outc)
+        error("output container '$outc' does not exist. use `mkpath to create it.")
+    end
+
     if Sys.iswindows()
         bytes = read!(in, Vector{UInt8}(undef, filesize(in)))
         write(outc, outb, bytes)

test/runtests.jl

@@ -502,6 +502,7 @@ end
     write("foolocal.txt", "Hello world")
     r = uuid4()
     c = AzContainer("foo-$r-o", storageaccount=storageaccount, session=session, nthreads=2, nretry=10)
+    @test_throws "does not exist" cp("foolocal.txt", c, "foo.txt")
     robust_mkpath(c)
     cp("foolocal.txt", c, "foo.txt")
     @test read(c, "foo.txt", String) == "Hello world"