Consistent use of err() in library code (#12902)

We had some explict references to `Module.printErr` and even
`Modue.printError` (which doesn't exist).   `Module.printErr`
is the external name which users can use to override `err`
but interally I think we should always prefer `err`.

Also rename a couple of local variables to avoid name colition
with printErr.

Author: sbc100

src/library_fs.js

@@ -1329,7 +1329,7 @@ FS.staticInit();` +
       });
       FS.mkdev('/dev/null', FS.makedev(1, 3));
       // setup /dev/tty and /dev/tty1
-      // stderr needs to print output using Module['printErr']
+      // stderr needs to print output using err() rather than out()
       // so we register a second tty just for it.
       TTY.register(FS.makedev(5, 0), TTY.default_tty_ops);
       TTY.register(FS.makedev(6, 0), TTY.default_tty1_ops);

src/library_pthread.js

@@ -651,7 +651,7 @@ var LibraryPThread = {
           }
           if (canvas.transferControlToOffscreen) {
 #if GL_DEBUG
-            Module['printErr']('pthread_create: canvas.transferControlToOffscreen(), transferring canvas by name "' + name + '" (DOM id="' + canvas.id + '") from main thread to pthread');
+            err('pthread_create: canvas.transferControlToOffscreen(), transferring canvas by name "' + name + '" (DOM id="' + canvas.id + '") from main thread to pthread');
 #endif
             // Create a shared information block in heap so that we can control
             // the canvas size from any thread.
@@ -679,7 +679,7 @@ var LibraryPThread = {
             // be able to transfer control to offscreen, but WebGL can be
             // proxied from worker to main thread.
 #if !OFFSCREEN_FRAMEBUFFER
-            Module['printErr']('pthread_create: Build with -s OFFSCREEN_FRAMEBUFFER=1 to enable fallback proxying of GL commands from pthread to main thread.');
+            err('pthread_create: Build with -s OFFSCREEN_FRAMEBUFFER=1 to enable fallback proxying of GL commands from pthread to main thread.');
             return {{{ cDefine('ENOSYS') }}}; // Function not implemented, browser doesn't have support for this.
 #endif
           }

src/library_webgl.js

@@ -3766,18 +3766,18 @@ var LibraryGL = {
     var mapping = GL.mappedBuffers[emscriptenWebGLGetBufferBinding(target)];
     if (!mapping) {
       GL.recordError(0x502 /* GL_INVALID_OPERATION */);
-      Module.printError('buffer was never mapped in glFlushMappedBufferRange');
+      err('buffer was never mapped in glFlushMappedBufferRange');
       return;
     }
 
     if (!(mapping.access & 0x10)) {
       GL.recordError(0x502 /* GL_INVALID_OPERATION */);
-      Module.printError('buffer was not mapped with GL_MAP_FLUSH_EXPLICIT_BIT in glFlushMappedBufferRange');
+      err('buffer was not mapped with GL_MAP_FLUSH_EXPLICIT_BIT in glFlushMappedBufferRange');
       return;
     }
     if (offset < 0 || length < 0 || offset + length > mapping.length) {
       GL.recordError(0x501 /* GL_INVALID_VALUE */);
-      Module.printError('invalid range in glFlushMappedBufferRange');
+      err('invalid range in glFlushMappedBufferRange');
       return;
     }
 
@@ -3800,7 +3800,7 @@ var LibraryGL = {
     var mapping = GL.mappedBuffers[buffer];
     if (!mapping) {
       GL.recordError(0x502 /* GL_INVALID_OPERATION */);
-      Module.printError('buffer was never mapped in glUnmapBuffer');
+      err('buffer was never mapped in glUnmapBuffer');
       return 0;
     }
     GL.mappedBuffers[buffer] = null;

src/postamble.js

@@ -321,8 +321,8 @@ function checkUnflushedContent() {
   // How we flush the streams depends on whether we are in SYSCALLS_REQUIRE_FILESYSTEM=0
   // mode (which has its own special function for this; otherwise, all
   // the code is inside libc)
-  var print = out;
-  var printErr = err;
+  var oldOut = out;
+  var oldErr = err;
   var has = false;
   out = err = function(x) {
     has = true;
@@ -349,8 +349,8 @@ function checkUnflushedContent() {
     });
 #endif
   } catch(e) {}
-  out = print;
-  err = printErr;
+  out = oldOut;
+  err = oldErr;
   if (has) {
     warnOnce('stdio streams had content in them that was not flushed. you should set EXIT_RUNTIME to 1 (see the FAQ), or make sure to emit a newline when you printf etc.');
 #if FILESYSTEM == 0 || SYSCALLS_REQUIRE_FILESYSTEM == 0