backtrace: add support for generic backtraces

Sometimes exceptions can contain manual backtraces that quack similar to
Ruby exceptions, but do not stricly adhere their template. I stumbled
upon this issues, when I was working with SCSS. It sets a couple of
custom first frames such as:

```
Sass::SyntaxError: Undefined mixin 'animation'.
/home/app/assets/stylesheets/error_pages.scss:139:in `animation'
/home/app/app/assets/stylesheets/error_pages.scss:139
```

Previously, we were not able to parse the second frame.

Furthermore, we now raise error if we cannot parse a stack frame to
simplify debugguging and improving our parser.

lib/airbrake-ruby/backtrace.rb

@@ -14,7 +14,7 @@ module Backtrace
     ##
     # @return [Regexp] the pattern that matches standard Ruby stack frames,
     #   such as ./spec/notice_spec.rb:43:in `block (3 levels) in <top (required)>'
-    STACKFRAME_REGEXP = %r{\A
+    RUBY_STACKFRAME_REGEXP = %r{\A
       (?<file>.+)       # Matches './spec/notice_spec.rb'
       :
       (?<line>\d+)      # Matches '43'
@@ -34,6 +34,16 @@ module Backtrace
       \)
     \z/x
 
+    ##
+    # @return [Regexp] the template that tries to assume what a generic stack
+    #   frame might look like, when exception's backtrace is set manually.
+    GENERIC_STACKFRAME_REGEXP = %r{\A
+      (?<file>.+)   # Matches '/foo/bar/baz.ext'
+      :
+      (?<line>\d+)  # Matches '43'
+      (?<function>) # No-op
+    \z}x
+
     ##
     # Parses an exception's backtrace.
     #
@@ -44,11 +54,11 @@ def self.parse(exception)
       regexp = if java_exception?(exception)
                  JAVA_STACKFRAME_REGEXP
                else
-                 STACKFRAME_REGEXP
+                 RUBY_STACKFRAME_REGEXP
                end
 
       (exception.backtrace || []).map do |stackframe|
-        stack_frame(regexp.match(stackframe))
+        stack_frame(match_frame(regexp, stackframe))
       end
     end
 
@@ -71,5 +81,16 @@ def stack_frame(match)
           function: match[:function] }
       end
     end
+
+    def self.match_frame(regexp, stackframe)
+      match = regexp.match(stackframe)
+      return match if match
+
+      unless (matched_frame = GENERIC_STACKFRAME_REGEXP.match(stackframe))
+        raise Airbrake::Error, "failed parsing '#{stackframe}'"
+      end
+
+      matched_frame
+    end
   end
 end

spec/backtrace_spec.rb

@@ -73,5 +73,40 @@
           to eq(backtrace_array)
       end
     end
+
+    context "generic backtrace" do
+      # rubocop:disable Metrics/LineLength
+      let(:generic_bt) do
+        ["/home/bingo/bango/assets/stylesheets/error_pages.scss:139:in `animation'",
+         "/home/bingo/bango/assets/stylesheets/error_pages.scss:139",
+         "/home/bingo/.gem/ruby/2.2.2/gems/sass-3.4.20/lib/sass/tree/visitors/perform.rb:349:in `block in visit_mixin'"]
+      end
+      # rubocop:enable Metrics/LineLength
+
+      let(:ex) { AirbrakeTestError.new.tap { |e| e.set_backtrace(generic_bt) } }
+
+      let(:parsed_backtrace) do
+        # rubocop:disable Metrics/LineLength, Style/HashSyntax, Style/SpaceInsideHashLiteralBraces, Style/SpaceAroundOperators
+        [{:file=>"/home/bingo/bango/assets/stylesheets/error_pages.scss", :line=>139, :function=>"animation"},
+         {:file=>"/home/bingo/bango/assets/stylesheets/error_pages.scss", :line=>139, :function=>""},
+         {:file=>"/home/bingo/.gem/ruby/2.2.2/gems/sass-3.4.20/lib/sass/tree/visitors/perform.rb", :line=>349, :function=>"block in visit_mixin"}]
+        # rubocop:enable Metrics/LineLength, Style/HashSyntax, Style/SpaceInsideHashLiteralBraces, Style/SpaceAroundOperators
+      end
+
+      it "returns a properly formatted array of hashes" do
+        expect(described_class.parse(ex)).to eq(parsed_backtrace)
+      end
+    end
+
+    context "unknown backtrace" do
+      let(:unknown_bt) { ['a b c 1 23 321 .rb'] }
+
+      let(:ex) { AirbrakeTestError.new.tap { |e| e.set_backtrace(unknown_bt) } }
+
+      it "raises error" do
+        expect { described_class.parse(ex) }.
+          to raise_error(Airbrake::Error, /failed parsing/)
+      end
+    end
   end
 end