ZIP: replace unused std::runtime_error("...") with an error print

We could have thrown an exception here, but it is better for library
code to be resilient and print a warning instead so that users can
rectify things without crashing their app.

Closes: #112

Author: davvid

src/lib/io/ZIP.cpp

@@ -399,7 +399,10 @@ class ZipStreambufCompress:public std::streambuf
     {if(pptr() && pptr()>pbase()) return process(false);return 0;}
 
     virtual int underflow()
-    {std::runtime_error("Attempt to read write only ostream");return 0;}
+    {
+        std::cerr << "error: Partio::ZipStreambufCompress attempted to read a write-only ostream" << std::endl;
+        return 0;
+    }
 
     virtual int overflow(int c=EOF)
     {if(c!=EOF){*pptr()=static_cast<char>(c);pbump(1);}