Correct location and format of flow expression parse errors

Take this example:

import org.checkerframework.checker.nullness.qual.*;

class ExpressionParse {
  @RequiresNonNull("kdfj")
  @EnsuresNonNull("skdfj")
  void bar() {}
}

Running the Nullness Checker gives:

ExpressionParse.java:6: error: [flowexpr.parse.error] cannot parse the expression 'kdfj: identifier not found'
  void bar() {}
       ^
ExpressionParse.java:6: error: [flowexpr.parse.error] cannot parse the expression 'kdfj: identifier not found'
  void bar() {}
       ^
ExpressionParse.java:6: error: [flowexpr.parse.error] cannot parse the expression 'skdfj: identifier not found'
  void bar() {}
       ^
3 errors

(The duplicate pre-condition error is being tracked as https://github.com/typetools/checker-framework/issues/1051)

Some possible improvements:

• It would be useful if the error message were on the line where the invalid String is and not always on the method declaration.
• It might also be useful to output more context, e.g. within which annotation the expression was.
• The message is rather confusingly presented: the code didn't try to parse skdfj: identifier not found; it tried to parse skdfj and that failed because that identifier wasn't found.

Start by reproducing the example with the Nullness Checker and the @RequiresNonNull and @EnsuresNonNull annotations shown in ExpressionParse.java. Trace how the flow expression parse error is reported, then define completion as reporting the error at the invalid String, including useful annotation context, and presenting the failed expression and cause clearly.