Suggest possible solutions to failing requires

spec/compiler/crystal_path/crystal_path_spec.cr

@@ -95,4 +95,20 @@ describe Crystal::CrystalPath do
   assert_doesnt_find "./crystal_path_spec", relative_to: "test_files/file_one.cr"
   assert_doesnt_find "./crystal_path_spec.cr", relative_to: "test_files/file_one.cr"
   assert_doesnt_find "../crystal_path/test_files/file_one"
+
+  it "prints an explanatory message for non-relative requires" do
+    crystal_path = Crystal::CrystalPath.new(__DIR__)
+    ex = expect_raises Exception, /If you're trying to require a shard/ do
+      crystal_path.find "non_existent", relative_to: __DIR__
+    end
+  end
+
+  it "doesn't print an explanatory message for relative requires" do
+    crystal_path = Crystal::CrystalPath.new(__DIR__)
+    ex = expect_raises Exception, /can't find file/ do
+      crystal_path.find "./non_existant", relative_to: __DIR__
+    end
+
+    ex.message.not_nil!.should_not contain "If you're trying to require a shard"
+  end
 end

src/compiler/crystal/crystal_path.cr

@@ -168,11 +168,21 @@ module Crystal
     end
 
     private def cant_find_file(filename, relative_to)
-      if relative_to
-        raise Error.new("can't find file '#{filename}' relative to '#{relative_to}'")
+      error = "can't find file '#{filename}'"
+
+      if filename.starts_with? '.'
+        error += " relative to '#{relative_to}'" if relative_to
       else
-        raise Error.new("can't find file '#{filename}'")
+        error = <<-NOTE
+          #{error}
+
+          If you're trying to require a shard:
+          - Did you remember to run `shards install`?
+          - Did you make sure you're running the compiler in the same directory as your shard.yml?
+          NOTE
       end
+
+      raise Error.new(error)
     end
   end
 end