Pass REXML exception messages onto the caller

Fixes #178

lib/prawn/svg/document.rb

@@ -18,18 +18,9 @@ class Prawn::SVG::Document
 
   def initialize(data, bounds, options, font_registry: nil, css_parser: CssParser::Parser.new, attribute_overrides: {})
     begin
-      @root = REXML::Document.new(data).root
-    rescue REXML::ParseException
-      @root = nil
-    end
-
-    if @root.nil?
-      if data.respond_to?(:end_with?) && data.end_with?('.svg')
-        raise InvalidSVGData,
-          "The data supplied is not a valid SVG document.  It looks like you've supplied a filename instead; use IO.read(filename) to get the data before you pass it to prawn-svg."
-      else
-        raise InvalidSVGData, 'The data supplied is not a valid SVG document.'
-      end
+      @root = REXML::Document.new(data).root or raise_parse_error(data)
+    rescue REXML::ParseException => e
+      raise_parse_error(data, e.message)
     end
 
     @warnings = []
@@ -73,4 +64,16 @@ def load_color_mode
       raise ArgumentError, ':color_mode must be set to :rgb (default) or :cmyk'
     end
   end
+
+  def raise_parse_error(data, exception_message = nil)
+    message = 'The data supplied is not a valid SVG document.'
+
+    if data.respond_to?(:end_with?) && data.end_with?('.svg')
+      message += "  It looks like you've supplied a filename instead; use File.read(filename) to get the data from the file before you pass it to prawn-svg."
+    end
+
+    message += "\n#{exception_message}" if exception_message
+
+    raise InvalidSVGData, message
+  end
 end

spec/prawn/svg/document_spec.rb

@@ -25,21 +25,35 @@
     context 'when unparsable XML is provided' do
       let(:svg) { "this isn't SVG data" }
 
-      it 'raises an exception' do
+      it "raises an exception, passing on REXML's error message" do
         expect do
           Prawn::SVG::Document.new(svg, bounds, options)
-        end.to raise_error Prawn::SVG::Document::InvalidSVGData, 'The data supplied is not a valid SVG document.'
+        end.to raise_error Prawn::SVG::Document::InvalidSVGData,
+          /\AThe data supplied is not a valid SVG document.+Malformed.+#{Regexp.escape(svg)}/m
+      end
+    end
+
+    context 'when broken XML is provided' do
+      let(:svg) { '<svg><g><rect></rect></svg>' }
+
+      it "raises an exception, passing on REXML's error message" do
+        expect do
+          Prawn::SVG::Document.new(svg, bounds, options)
+        end.to raise_error Prawn::SVG::Document::InvalidSVGData,
+          /\AThe data supplied is not a valid SVG document.+Missing end tag for 'g'/m
       end
     end
 
     context 'when the user passes in a filename instead of SVG data' do
       let(:svg) { 'some_file.svg' }
 
       it "raises an exception letting them know what they've done" do
+        message = "The data supplied is not a valid SVG document.  It looks like you've supplied a filename " \
+                  'instead; use File.read(filename) to get the data from the file before you pass it to prawn-svg.'
+
         expect do
           Prawn::SVG::Document.new(svg, bounds, options)
-        end.to raise_error Prawn::SVG::Document::InvalidSVGData,
-          "The data supplied is not a valid SVG document.  It looks like you've supplied a filename instead; use IO.read(filename) to get the data before you pass it to prawn-svg."
+        end.to raise_error Prawn::SVG::Document::InvalidSVGData, /\A#{Regexp.escape(message)}/
       end
     end
   end