Check enitity aliases before execution

[shytikov]

FXB is not validating entity aliases during operation.

For example following FetchXML will fail to execute due to fact that queueitem entity alias contain illegal characters, in this case it's dash (-).

<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false" >
  <entity name="incident" >
    <attribute name="createdon" />
    <attribute name="followupby" />
    <attribute name="subjectid" />
    <filter type="and" >
      <filter type="or" >
        <condition attribute="ownerid" operator="eq-userid" />
      </filter>
      <condition attribute="statecode" operator="eq" value="0" />
    </filter>
    <attribute name="incidentid" />
    <order attribute="createdon" descending="true" />
    <link-entity name="queueitem" from="objectid" to="incidentid" link-type="inner" alias="queue-item" >
      <link-entity name="queue" from="queueid" to="queueid" link-type="inner" alias="queue" >
        <attribute name="name" alias="Queue" />
      </link-entity>
    </link-entity>
  </entity>
</fetch>

FXB fires reasonably logical error, but very broad error:

---------------------------
Execute
---------------------------
Invalid character specified for alias: queue-item. Only characters within the ranges [A-Z], [a-z] or [0-9] or _ are allowed.  The first character may only be in the ranges [A-Z], [a-z] or _.

Try with result as ExecuteFetch?
---------------------------
OK   Cancel   
---------------------------

Through it would be much nicer if such error as incorrect alias would be detected before execution started. This will help user to fix error more quickly. And this is especially important for quite large FetchXML requests.

[rappen]

Related to #106.
But I have doubts about implementing too specific validation, for two reasons:

1. The rule causing this error might very well change in later releases of CRM.
2. The error message given by CRM is already well formed and detailed enough to fix the problem.

That said, this may still be implemented as part of the more extensive validation in related issue.