Add context to file load failures in pdfium_test.

samples/pdfium_test.cc

Index: samples/pdfium_test.cc
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 66dd3f2f1bc461e807e98b610ae03656c081e434..0a1693ac7a9fa5dc86d3e28eedf9436ebd310da4 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -390,7 +390,35 @@ void RenderPdf(const std::string& name, const char* pBuf, size_t len,
   }
 
   if (!doc) {
-    fprintf(stderr, "Load pdf docs unsuccessful.\n");
+    unsigned long err = FPDF_GetLastError();
+    fprintf(stderr, "Load pdf docs unsuccessful: ");
+    switch (err) {
+      case FPDF_ERR_SUCCESS:
+        fprintf(stderr, "Success");
+        break;
+      case FPDF_ERR_UNKNOWN:
+        fprintf(stderr, "Unknown error");
+        break;
+      case FPDF_ERR_FILE:
+        fprintf(stderr, "File not found or could not be opened");
+        break;
+      case FPDF_ERR_FORMAT:
+        fprintf(stderr, "File not in PDF format or corrupted");
+        break;
+      case FPDF_ERR_PASSWORD:
+        fprintf(stderr, "Password required or incorrect password");
+        break;
+      case FPDF_ERR_SECURITY:
+        fprintf(stderr, "Unsupported security scheme");
+        break;
+      case FPDF_ERR_PAGE:
+        fprintf(stderr, "Page not found or content error");
+        break;
+      default:
+        fprintf(stderr, "Unknown error %ld", err);
+    }
+    fprintf(stderr, ".\n");
+
     FPDFAvail_Destroy(pdf_avail);
     return;
   }