more informative error message when JUPYTER is not a valid executable

deps/build.jl

@@ -13,27 +13,40 @@ try
 ENV["PYTHONIOENCODING"] = "UTF-8"
 
 function prog_version(prog)
+    v = try
+        chomp(read(`$prog --version`, String))
+    catch
+        return nothing
+    end
     try
-       return convert(VersionNumber, chomp(read(`$prog --version`, String)))
+       return convert(VersionNumber, v)
     catch
-       return v"0.0"
+        warn("`$jupyter --version` returned an unrecognized version number $v")
+        return v"0.0"
     end
 end
 
 global jupyter = get(ENV, "JUPYTER", isfile("JUPYTER") ? readchomp("JUPYTER") : Compat.Sys.islinux() ? "jupyter" : "")
-jupyter_vers = isempty(jupyter) ? v"0.0" : prog_version(jupyter)
-if (jupyter_vers == v"0.0")                                                     # some Linux distributions (Debian) use jupyter-notebook to launch Jupyter
-    jupyter_vers = prog_version(jupyter * "-notebook")
+if isempty(jupyter)
+    jupyter_vers = nothing
+else
+    jupyter_vers = prog_version(jupyter)
+    if jupyter_vers === nothing
+        jupyter_vers = prog_version(jupyter * "-notebook")
+    end
+    if jupyter_vers === nothing
+        warn("Could not execute `$jupyter --version`.")
+    end
 end
 isconda = dirname(jupyter) == abspath(Conda.SCRIPTDIR)
-if Sys.ARCH in (:i686, :x86_64) && (jupyter_vers < v"3.0" || isconda)
-    isempty(jupyter) || isconda || info("$jupyter was too old: got $jupyter_vers, required ≥ 3.0")
+if Sys.ARCH in (:i686, :x86_64) && (jupyter_vers === nothing || jupyter_vers < v"3.0" || isconda)
+    isconda || jupyter_vers === nothing || info("$jupyter was too old: got $jupyter_vers, required ≥ 3.0")
     info("Installing Jupyter via the Conda package.")
     Conda.add("jupyter")
     jupyter = abspath(Conda.SCRIPTDIR, "jupyter")
     jupyter_vers = prog_version(jupyter)
 end
-if jupyter_vers < v"3.0"
+if jupyter_vers === nothing || jupyter_vers < v"3.0"
     error("Failed to find or install Jupyter 3.0 or later. Please install Jupyter manually, set `ENV[\"JUPYTER\"]=\"/path/to/jupyter\", and rerun `Pkg.build(\"IJulia\")`.")
 end
 info("Found Jupyter version $jupyter_vers: $jupyter")